Extract subscription module into submodules

[ryanoneill]

Summary

• Splits the 1000-line src/app/subscription/mod.rs into 6 focused submodules
• core.rs (258 lines): Subscription trait, BoxedSubscription, Tick/Timer/Channel/Stream subscriptions
• combinators.rs (366 lines): Mapped, Filter, Take, Debounce, Throttle subscriptions
• ext.rs (128 lines): SubscriptionExt trait with combinator methods
• batch.rs (44 lines): BatchSubscription and batch() function
• interval.rs (107 lines): IntervalImmediateSubscription and interval_immediate()
• terminal.rs (112 lines): TerminalEventSubscription and terminal_events()
• New mod.rs is 42 lines (re-exports only), down from 1000
• All public API paths preserved — zero breaking changes

Test plan

• cargo test --all-features passes (2882 unit + 267 doc)
• cargo clippy --all-features -- -D warnings clean
• All 59 subscription tests pass unchanged

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

❌ Patch coverage is 85.95506% with 25 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.82%. Comparing base (ff80ab6) to head (f26e224).
⚠️ Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
src/app/subscription/terminal.rs	12.50%	14 Missing ⚠️
src/app/subscription/combinators.rs	92.00%	6 Missing ⚠️
src/app/subscription/core.rs	95.83%	2 Missing ⚠️
src/app/subscription/interval.rs	89.47%	2 Missing ⚠️
src/app/subscription/batch.rs	90.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #106   +/-   ##
=======================================
  Coverage   92.82%   92.82%           
=======================================
  Files          94       99    +5     
  Lines        9471     9471           
=======================================
  Hits         8791     8791           
  Misses        680      680           

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