Deprecate TailStrategy::Predicate in favor of GuardWithIf

[mcourteaux]

Summary

First step of the deprecation plan in #9443:

• TailStrategy::Predicate now shares the same underlying enum value as TailStrategy::GuardWithIf (they have been behaviorally identical in ApplySplit.cpp for a while) and is marked [[deprecated]].
• Switch statements over TailStrategy (ApplySplit.cpp, IRPrinter.cpp, Serialization.cpp) no longer need a separate case for Predicate, since it now compiles to the same value as GuardWithIf.
• Deserialization.cpp maps old serialized Predicate pipelines to GuardWithIf directly, without referencing the deprecated identifier.
• Func.cpp's exact-split legality check is simplified accordingly.
• The Python binding keeps exposing TailStrategy.Predicate for this release (with the deprecation warning locally suppressed), per the plan to remove it for real only in the release after next.
• Updated the internal uses of TailStrategy::Predicate in a handful of tests and the hannk app to use GuardWithIf instead, so the codebase doesn't trigger its own new deprecation warning.

Closes #9443 (partially — the follow-up release will remove Predicate entirely).

Test plan

• libHalide builds clean (no warnings) with these changes
• Serialization.cpp, Deserialization.cpp, and PyEnums.cpp individually syntax-checked with -Wdeprecated-declarations: none triggered
• All touched test/app files compile
• clang-format --dry-run --Werror clean on all touched files
• pre-commit run --all-files passed on the commit

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.93%. Comparing base (0b4fbea) to head (fae150a).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/Func.cpp	0.00%	1 Missing and 1 partial ⚠️
src/Deserialization.cpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9446      +/-   ##
==========================================
+ Coverage   69.84%   69.93%   +0.09%     
==========================================
  Files         261      261              
  Lines       79615    79605      -10     
  Branches    19402    19398       -4     
==========================================
+ Hits        55606    55674      +68     
+ Misses      18042    18013      -29     
+ Partials     5967     5918      -49     

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