feat: add block-scoped modifiers

[invokermain]

Summary

• Block-scoped modifiers: Modify declared inside a Block now only affects resolutions within that block's scope, matching fx/dig's scoped decorator behavior. Modifiers in BlockA don't leak to BlockB or top-level invocations.
• Modifier composition: Block modifiers compose with global modifiers — a block modifier receives the globally-modified value as input (e.g., global prefix + block uppercase = "GLOBAL_FOO").
• Bug fix: _bind_arguments (used by assemble()) now applies modifiers. Previously, invocations assembled via assemble() received raw provider values, bypassing all modifiers.

Implementation details

• _ScopeNode extended with modifiers dict and find_modifier() chain walk (mirrors dig's storesToRoot())
• Global modifiers moved from Assembler._modifiers onto root_node.modifiers
• Block scope nodes are persistent _ScopeNode instances parented to root, created during Block.apply()
• Engin.run() sets _scope_var to the block's node when assembling/invoking block invocations
• Instance-level on-stack tracking (set[int] of modifier IDs) replaces type-level tracking to allow modifier composition across scope levels
• Modified values cached on the modifier's source node (not always root) to prevent cross-scope leaking
• find_modified() stops at nodes owning a modifier for the type, preventing inheritance of parent's cached modified value

Test plan

• Block modifier only affects block's own invocations
• Block modifier does not affect other blocks
• Block + global modifier composition (block receives globally-modified value)
• Global modifier works through Engin invocation path (not just Assembler.build)
• All 143 existing + new tests pass
• poe check passes (ruff format, ruff lint, mypy)

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 87.12121% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.72%. Comparing base (29f675e) to head (e769660).

Files with missing lines	Patch %	Lines
src/engin/_engin.py	88.37%	4 Missing and 1 partial ⚠️
src/engin/exceptions.py	44.44%	5 Missing ⚠️
src/engin/_assembler.py	93.54%	3 Missing and 1 partial ⚠️
src/engin/_dependency.py	70.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #72      +/-   ##
==========================================
- Coverage   91.29%   90.72%   -0.57%     
==========================================
  Files          19       19              
  Lines        1309     1402      +93     
  Branches      184      201      +17     
==========================================
+ Hits         1195     1272      +77     
- Misses         69       83      +14     
- Partials       45       47       +2     

Flag	Coverage Δ
macos-latest	89.94% <87.12%> (-0.51%)	⬇️
py3.10	90.37% <87.12%> (-0.54%)	⬇️
py3.11	90.37% <87.12%> (-0.54%)	⬇️
py3.12	90.37% <87.12%> (-0.54%)	⬇️
py3.13	90.37% <87.12%> (-0.54%)	⬇️
py3.14	90.37% <87.12%> (-0.55%)	⬇️
ubuntu-latest	89.94% <87.12%> (-0.51%)	⬇️
windows-latest	90.22% <87.12%> (-0.53%)	⬇️

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