feat: add T command (branch on no-substitution)

[MukundaKatta]

Summary

Implements the GNU T LABEL command (issue #395): branch to LABEL if no s/// substitution has succeeded since the last input line was read or the last t/T was taken. It is the inverse of t.

Before:

$ echo a | ./target/release/sed -n 'Tend; p; :end'
sed: <script argument 1>:1:1: error: invalid command code `T'

After:

$ echo a | ./target/release/sed -n 'Tend; p; :end'      # no s/// -> T branches over p
$ echo a | ./target/release/sed -n 's/a/X/; Tend; p; :end'
X                                                        # s/// ran -> T does not branch

Implementation

• get_cmd_spec: accept T alongside b/t (2 addresses, compile_label_command). Label parsing is already code-agnostic.
• resolve_branch_targets: include T when converting Label -> BranchTarget.
• Processor: T branches when !substitution_made; in both the branch and no-branch arms the flag is reset on evaluation, matching t.

Testing

• cargo test: full suite green (282 unit + 183 integration).
• cargo clippy --all-targets -- -D warnings and cargo fmt --check: clean.
• New branch_no_sub_simple (selective branch) and branch_no_sub_clears (proves T clears the substitution flag) integration tests, plus a get_cmd_spec unit test covering b/t/T.

Closes #395

[codecov]

Codecov Report

❌ Patch coverage is 42.10526% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.13%. Comparing base (7c6cbf9) to head (8903013).

Files with missing lines	Patch %	Lines
src/sed/processor.rs	0.00%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #448      +/-   ##
==========================================
- Coverage   82.20%   82.13%   -0.08%     
==========================================
  Files          13       13              
  Lines        5542     5559      +17     
  Branches      310      312       +2     
==========================================
+ Hits         4556     4566      +10     
- Misses        983      990       +7     
  Partials        3        3              

Flag	Coverage Δ
macos_latest	82.82% <42.10%> (-0.08%)	⬇️
ubuntu_latest	82.92% <42.10%> (-0.08%)	⬇️
windows_latest	0.00% <0.00%> (ø)

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