Convert simulation IR to frozen ccflow models

[timkpaine]

The largest single cluster in the standards ledger: the IR's 32 node types (expr.py, stmt.py, module.py, validate.py) plus SimulationResult and VerilatorTestbenchResult were standalone dataclasses. The IR is this package's public API — it crosses the builder, validator, lowering and simulation boundaries — so these are spec types under the standard.

All convert to frozen ccflow BaseModels. Verified independently: 32 IR models, all frozen, zero dataclasses remaining, mutation raises.

• recursive Expr/Stmt/Module typing preserved, with polymorphic round-trip tests rather than loosening to Any
• 642 positional construction sites across 16 files became keyword calls
• mutable containers → tuples; ValidationResult now accumulates locally and constructs one frozen result
• no per-cycle IR construction exists, so the conversion cost is bounded to attribute reads

Honest cost, measured: simulation throughput drops 5.5% (337,843 → 319,213 cycles/s); compile median 0.244 → 0.250 ms. That is pydantic attribute access in the interpreter walk. Reported rather than hidden — if it matters, the interpreter can snapshot hot fields into locals, but that is an optimization for its own change.

Suite: 504 passed. Clears ~35 of the 68 ledger violations.

[github-actions]

Test Results

505 tests  ±0   502 ✅ ±0   24s ⏱️ +10s
  2 suites ±0     3 💤 ±0 
  2 files   ±0     0 ❌ ±0 

Results for commit 3f5f874. ± Comparison against base commit 9b2aaee.

[codecov]

Codecov Report

❌ Patch coverage is 88.74172% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.54%. Comparing base (9b2aaee) to head (3f5f874).

Files with missing lines	Patch %	Lines
dau_sim/ir/validate.py	60.52%	15 Missing ⚠️
dau_sim/api.py	93.75%	1 Missing ⚠️
dau_sim/ir/printer.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #52      +/-   ##
==========================================
- Coverage   84.56%   84.54%   -0.02%     
==========================================
  Files          60       60              
  Lines        9712     9702      -10     
  Branches     1240     1240              
==========================================
- Hits         8213     8203      -10     
  Misses       1198     1198              
  Partials      301      301              

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