Merge repeated Func bound/align_bounds/align_extent calls per Var

[mcourteaux]

Func::bound(), align_bounds(), and align_extent() each appended an independent Bound entry to FuncSchedule::bounds(), even when a Bound for that Var already existed. With two entries for the same Var, every consumer of bounds() (BoundsInference, ScheduleFunctions, AllocationBoundsInference) had to guess how to combine them, and did so inconsistently -- most notably, BoundsInference's LetStmt-based composition ends up applying the entries in the reverse of the order they were pushed, which is why the call order between bound_extent() and align_bounds() mattered and, when reversed, made bounds inference derive a too-small region (a runtime "do not cover required region" failure), not just leftover unsimplified expressions.

Fold repeated calls for the same Var into a single Bound instead, with a user_warning when a call overwrites a field a previous call already set. Every consumer now sees at most one Bound per Var, so call order no longer matters.

Breaking changes

None.

Checklist

• Tests added or updated (not required for docs, CI config, or typo fixes)
• Documentation updated (if public API changed)
• Python bindings updated (if public API changed)
• Benchmarks are included here if the change is intended to affect performance.
• Commits include AI attribution where applicable (see Code of Conduct)

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.05%. Comparing base (d9debf9) to head (df3e828).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/Inline.cpp	0.00%	1 Missing and 1 partial ⚠️
src/Schedule.cpp	89.47%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9410      +/-   ##
==========================================
+ Coverage   69.95%   70.05%   +0.09%     
==========================================
  Files         261      261              
  Lines       79402    79424      +22     
  Branches    19360    19365       +5     
==========================================
+ Hits        55546    55639      +93     
+ Misses      17932    17929       -3     
+ Partials     5924     5856      -68     

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[abadams]

LGTM but the linux-32 failure looks like it might be real. Dropping a bound could slow that down. Could also be a flake though. I'll rerun it.