Add simplify_broadcasted as the supported downstream entry point

[jishnub]

Stacked on #385 — review that first; this PR's diff is the single commit on top.

The problem

A package with a BroadcastStyle of its own currently reaches the fill rules by forwarding through broadcast(DefaultArrayStyle{N}(), op, args...). That protocol is undocumented and leaks implementation details three ways:

1. It names a style that has nothing to do with fills — it is merely where the rules used to be attached.
2. It makes the caller enumerate the argument shapes that happen to have rules. Miss one and the simplification is silently lost; forward one we don't handle and the result materializes densely. LazyArrays documents this hazard in a comment.
3. It requires knowing that x .^ k lowers to a three-argument Base.literal_pow with Ref arguments, and that our rule takes them unwrapped. Both LazyArrays and InfiniteArrays carry near-duplicate methods for this alone.

Packages also call internals directly: _broadcasted_zeros (BlockArrays, BandedMatrices), mult_zeros (ArrayLayouts, LazyArrays, QuasiArrays), elconvert, _range_convert, steprangelen.

The change

FillArrays.simplify_broadcasted(style, op, args...)

Returns the simplified array where a rule applies, and otherwise a Broadcasted carrying the caller's own style — so anything we cannot simplify comes back for the caller to keep lazy, infinite, or on-device. Ref-wrapped literal_pow arguments are unwrapped, so callers never mention it.

This is the existing _dispatch_on_fills with two changes: the fallback style is parameterized instead of hard-coded to DefaultArrayStyle{N}, and the Ref-unwrapping folds in. The dimension now comes from the arguments rather than the style, so an AbstractArrayStyle{Any} caller works too.

Resolution order is unchanged, and for the same reason as before: the Zeros-absorbing rules are styleless and would otherwise be preempted by the more specific FillStyle range rules.

1. has_fill_rule → styleless rule
2. broadcasted(FillStyle{N}(), …) → fill-style rule
3. isfill → evaluate on the fill values
4. Broadcasted{typeof(style)}

The DefaultArrayStyle shims now delegate here, leaving one implementation rather than two. They are kept for packages that predate this.

Verification

• A 3592-probe sweep of broadcasted(DefaultArrayStyle{N}(), op, args...) is byte-identical before and after, so unmigrated packages are unaffected.
• Full suite passes; Broadcast testset 1524 → 1533 with the new tests, covering all four resolution branches including that the fallback carries the caller's style.

Downstream, against this branch: LazyArrays 35 broadcast methods → 14, InfiniteArrays' 4 redundant literal_pow methods → 0, BlockArrays and BandedMatrices 4 → 3 each with _broadcasted_zeros dropped. All four suites pass, and a 16-probe old-vs-new diff against their real checkouts shows no behavioural change.

Versioning

This is a minor release (1.19.0): simplify_broadcasted is new, the DefaultArrayStyle
shims are untouched in behaviour, and the 3592-probe sweep over them is byte-identical. Every
published downstream keeps resolving — LazyArrays and InfiniteArrays pin FillArrays = "1.0",
BlockArrays "1.11", BandedMatrices "1.3", and all of those admit 1.19. Packages opt into the
new entry point by raising their lower bound to "1.19" when they are ready; nothing is forced.

Deleting the DefaultArrayStyle shims (src/fillbroadcast.jl:245-254) is the eventual endpoint of
this migration and is genuinely breaking. That is deliberately left for a future major release,
once downstream packages have moved over.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.63%. Comparing base (b7eeafb) to head (5e01e70).

Files with missing lines	Patch %	Lines
src/fillbroadcast.jl	88.88%	1 Missing ⚠️

Additional details and impacted files

@@                    Coverage Diff                     @@
##           jishnub/broadcaststyle     #444      +/-   ##
==========================================================
+ Coverage                   93.60%   93.63%   +0.03%     
==========================================================
  Files                           9        9              
  Lines                        1298     1305       +7     
==========================================================
+ Hits                         1215     1222       +7     
  Misses                         83       83              

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