feat(flutter): Add standalone app start tracing

[buenaflor]

Adds opt-in standalone native app-start tracing for Flutter. With
options.enableStandaloneAppStartTracing enabled, native app start is emitted
as a detached App Start transaction with op app.start and origin
auto.app.start; with the flag disabled, native app start remains attached to
the initial ui.load transaction.

Architecture

This keeps each telemetry signal under one owner:

• TimeToDisplayTracker / TimeToDisplayTrackerV2 own the initial ui.load
  root, TTID/TTFD, currentDisplay, and reportFullyDisplayed compatibility.
• StaticStandaloneAppStartEmitter / StreamStandaloneAppStartEmitter own
  only the detached App Start / app.start root.
• NativeAppStartIntegration owns native source orchestration: first-frame
  callback registration, duplicate processing guard, native fetch/parse, display
  branch invocation, standalone branch invocation, and failure cleanup.
• App-start span writers own formatting only: legacy attached measurement and
  breakdown shape, standalone breakdown shape, app-start type data/attributes,
  and finalization-time app-start vitals encoding.

In standalone mode, the ui.load root still tracks TTID/TTFD but no longer
carries the app-start measurement or breakdown spans. The standalone root keeps
the current public shape:

App Start                                 op: app.start
├─ <native breakdown spans>               op: app.start.native
├─ App start to plugin registration       op: app.start.plugin_registration
├─ Before Sentry Init Setup               op: app.start.sentry_setup
└─ First frame render                     op: app.start.first_frame_render

The static standalone root deliberately does not use trimEnd: the explicit
native app-start end timestamp is authoritative, and trimming could let an
out-of-range native span stretch the transaction duration beyond the app-start
measurement.

Scope

This PR is native app start only. Generic standalone app start remains future
work and should reuse the same display-tracker entrypoints and standalone
emitter seam when conventions can represent its app-start type honestly.

#3767 extended app start remains follow-up work. The emitter seam is intended to
support that later by letting the standalone finalization policy evolve; a
session/tracker should be added then only if it owns real lifecycle state such
as extension first-wins behavior, deadline handling, or delayed finalization.

Fixes #3634
Refs #3767

[github-actions]

Semver Impact of This PR

🟡 Minor (new features)

📋 Changelog Preview

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---

Features

• (flutter) Add standalone app start tracing by buenaflor in #3853

---

_{🤖 This preview updates automatically when you update the PR.}

[github-actions]

🚨 Detected changes in high risk code 🚨

High-risk code has higher potential to break the SDK and may be hard to test. To prevent severe bugs, apply the rollout process for releasing such changes and be extra careful when changing and reviewing these files:

• packages/flutter/lib/src/integrations/native_app_start_integration.dart

[codecov]

Codecov Report

❌ Patch coverage is 75.10730% with 58 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.23%. Comparing base (c652e19) to head (0e4d23d).
⚠️ Report is 3 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...ib/src/app_start/native_app_start_integration.dart	14.51%	53 Missing ⚠️
...ib/src/app_start/stream_app_start_span_writer.dart	96.15%	2 Missing ⚠️
...ages/flutter/lib/src/app_start/app_start_info.dart	90.90%	1 Missing ⚠️
...ter/lib/src/app_start/native_app_start_parser.dart	0.00%	1 Missing ⚠️
...ib/src/app_start/static_app_start_span_writer.dart	98.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3853      +/-   ##
==========================================
- Coverage   87.35%   87.23%   -0.12%     
==========================================
  Files         338      341       +3     
  Lines       12283    12365      +82     
==========================================
+ Hits        10730    10787      +57     
- Misses       1553     1578      +25     

Flag	Coverage Δ
sentry	87.48% <100.00%> (ø)
sentry_dio	97.73% <ø> (ø)
sentry_drift	93.57% <ø> (ø)
sentry_file	65.29% <ø> (ø)
sentry_firebase_remote_config	100.00% <ø> (ø)
sentry_flutter	90.61% <74.89%> (-0.49%)	⬇️
sentry_grpc	99.09% <ø> (ø)
sentry_hive	77.48% <ø> (ø)
sentry_isar	74.37% <ø> (ø)
sentry_link	21.50% <ø> (ø)
sentry_logging	97.01% <ø> (ø)
sentry_sqflite	88.81% <ø> (ø)
sentry_supabase	97.27% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

[github-actions]

iOS Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	1255.69 ms	1252.57 ms	-3.12 ms
Size	5.83 MiB	6.27 MiB	455.42 KiB

Baseline results on branch: main

Startup times

Revision	Plain	With Sentry	Diff
0f0a415	1228.28 ms	1233.51 ms	5.23 ms
5367726	1211.00 ms	1216.73 ms	5.73 ms
f80073a	1249.75 ms	1243.52 ms	-6.23 ms
d15266b	1247.42 ms	1251.29 ms	3.87 ms
2f63d89	1251.67 ms	1263.94 ms	12.27 ms
bfabaf2	1251.72 ms	1253.38 ms	1.67 ms
8af916c	1246.66 ms	1240.79 ms	-5.87 ms
9441966	1239.78 ms	1254.00 ms	14.22 ms
54acf91	1257.65 ms	1277.96 ms	20.31 ms
396cb30	1265.06 ms	1265.73 ms	0.67 ms

App size

Revision	Plain	With Sentry	Diff
0f0a415	5.83 MiB	6.28 MiB	458.16 KiB
5367726	5.73 MiB	6.17 MiB	455.87 KiB
f80073a	5.73 MiB	6.18 MiB	464.26 KiB
d15266b	5.83 MiB	6.28 MiB	462.36 KiB
2f63d89	5.65 MiB	6.09 MiB	446.25 KiB
bfabaf2	5.53 MiB	6.01 MiB	487.95 KiB
8af916c	5.73 MiB	6.18 MiB	463.84 KiB
9441966	5.83 MiB	6.28 MiB	459.62 KiB
54acf91	20.70 MiB	22.46 MiB	1.75 MiB
396cb30	5.53 MiB	6.01 MiB	488.17 KiB

Previous results on branch: feat/standalone-app-start

Startup times

Revision	Plain	With Sentry	Diff
5b17145	1247.70 ms	1250.09 ms	2.39 ms

App size

Revision	Plain	With Sentry	Diff
5b17145	5.83 MiB	6.27 MiB	456.23 KiB

[github-actions]

Android Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	364.41 ms	349.38 ms	-15.03 ms
Size	14.55 MiB	15.87 MiB	1.31 MiB

Baseline results on branch: main

Startup times

Revision	Plain	With Sentry	Diff
f872f8e	402.38 ms	416.20 ms	13.82 ms
de377fd	572.35 ms	589.21 ms	16.87 ms
944b773	470.54 ms	480.18 ms	9.64 ms
d0aa4b6	395.66 ms	393.13 ms	-2.53 ms
cba2765	405.98 ms	422.02 ms	16.04 ms
a34fde0	393.60 ms	389.96 ms	-3.65 ms
c741aa5	360.67 ms	367.86 ms	7.19 ms
79df88f	355.02 ms	354.26 ms	-0.76 ms
f3d7a42	358.26 ms	343.32 ms	-14.94 ms
dfcc88b	387.20 ms	387.70 ms	0.50 ms

App size

Revision	Plain	With Sentry	Diff
f872f8e	13.93 MiB	15.18 MiB	1.25 MiB
de377fd	6.54 MiB	7.71 MiB	1.17 MiB
944b773	13.93 MiB	15.00 MiB	1.06 MiB
d0aa4b6	13.93 MiB	15.18 MiB	1.25 MiB
cba2765	13.93 MiB	15.18 MiB	1.25 MiB
a34fde0	14.09 MiB	15.28 MiB	1.19 MiB
c741aa5	14.55 MiB	15.87 MiB	1.31 MiB
79df88f	14.55 MiB	15.87 MiB	1.31 MiB
f3d7a42	14.55 MiB	15.87 MiB	1.31 MiB
dfcc88b	14.55 MiB	15.87 MiB	1.31 MiB

Previous results on branch: feat/standalone-app-start

Startup times

Revision	Plain	With Sentry	Diff
5b17145	374.48 ms	371.14 ms	-3.34 ms

App size

Revision	Plain	With Sentry	Diff
5b17145	14.55 MiB	15.87 MiB	1.31 MiB

[buenaflor]

Will be replaced by another impl