…oupling finding (#1472)

Adds §50 documenting the architecture-mismatch finding caught after §49.6
steps 3+4 landed (PR #1470 contract + PR #1471 wire-up). The remaining
§49.6 step 5 was scoped at "0 LOC, just run apr pretrain --init" — that
assumption is empirically wrong.

Empirical finding (§50.1):
  pretrain_real.rs:38-46 HARDCODES Llama370MConfig::* for every
  architectural constant. Qwen2.5-Coder-0.5B-Instruct has different
  shape across the board:

    Param            | Llama370M | Qwen2.5-Coder-0.5B
    -----------------|-----------|--------------------
    hidden_size      | 1024      | 896
    num_attention_heads | 16     | 14
    num_kv_heads     | 4 (GQA-4:1) | 2 (GQA-7:1)
    intermediate_size | 2816    | 4864
    vocab_size       | 50_257    | 151_936
    rope_theta       | 10_000    | 1_000_000

  Every tensor mismatches. Loading Qwen2.5 weights into a Llama370M-
  shaped optimizer is a category error.

Three options surfaced (§50.3):
  A: Find/build a Llama-shaped 0.5B pretrained checkpoint
     (~5K LOC + multi-week training; recreates §24/§25 corpus problem)
  B: Make trainer architecture-polymorphic
     (~200-400 LOC; preserves §24/§25 falsification; recommended)
  C: Replace Llama370MConfig with Qwen2_5_Coder_0_5B_Config outright
     (~300 LOC; deletes a working falsification path)

Recommendation (§50.5): Option B — preserves §24/§25 falsification
evidence, exercises TransformerConfig's designed polymorphism, binds
each new component (qwen2_0_5b constructor, GQA-7:1 attention, Qwen
tokenizer surface) to its own falsifier.

Re-scoped roadmap (§50.4) — 8 sub-steps replacing original step 5:
  5a. Author apr-pretrain-arch-polymorphic-v1.yaml contract  (~80 LOC)
  5b. TransformerConfig::qwen2_0_5b() constructor           (~40 LOC)
  5c. Extract arch from init APR file metadata              (~80 LOC)
  5d. Qwen tokenizer-vocab compatibility check              (~30 LOC)
  5e. GQA-7:1 attention forward-pass verification           (~50 LOC)
  5f. Wire actual weight load                              (~120 LOC)
  5g. LIVE 500-step smoke fine-tune (operator dispatch)        0 LOC
  5h. Stamp + publish as MODEL-2 v2                         (~10 LOC)

  Total: ~410 LOC + 1 LIVE training run.

Five Whys (§50.6):
  1. Why didn't §49 catch this? §49 was authored from strategy/
     data-budget reasoning; the 0-LOC step-5 cost implicitly
     assumed polymorphism. Live source inspection (this section's
     empirical move) revealed pretrain_real.rs:38-46 predates the
     assumption.
  2. Why catch this NOW and not in step 5 implementation? Per
     feedback_no_guessing.md: read live source before forming
     implementation plan. Surfacing the mismatch BEFORE writing
     200 LOC of weight-load code that fails at runtime is the
     cheapest place to pay cost-of-defect. The §50-prior wrong-
     premise PRs (#1466/#1467/#1468 closed) on the SHIP-007 / 0.5B
     gibberish track were the same defect class.
  3. Why option B over A or C? Preserves §24/§25 falsification
     evidence (we KEEP knowing from-scratch fails at 9.75; we just
     don't ship it as MODEL-2). Exercises the polymorphism
     TransformerConfig was designed for. Each new component becomes
     its own falsifier rather than a hidden coupling.
  4. Why is FALSIFY-005 the right place to fail-fast? PR #1470
     already pinned "Architecture mismatch is FAIL-FAST, not silent-
     truncate". Step 4 (PR #1471) doesn't enforce arch matching yet
     — returns "not yet wired" before getting there. So FALSIFY-005
     is currently UNBOUND but its discharge gate is well-defined:
     read APR header, compare against pretrain target, error with
     names of mismatched fields.
  5. Why isn't this a "punt"? A punt would say "blocked, await
     operator". This amendment names three options with LOC
     estimates, recommends one with reasoning, gives a concrete 8-
     step roadmap with falsifier discharge mapped to each sub-step.
     The work IS shippable; it's just bigger than 0 LOC.

Plain ship-% update:
  - MODEL-1: unchanged at 91% (SHIP-007 cascade infrastructure track)
  - MODEL-2: unchanged at 57% — first ship-% movement gated on §50.4
    step 5g (LIVE 500-step fine-tune producing val_loss < 9.38).
    Sub-steps 5a-5f can each individually move 1% with falsifier
    discharge (architecture-polymorphic infrastructure shipped ==
    evidence that the §49 path is REACHABLE, not just theoretical).

Refs:
  - §49 — MODEL-2 strategy pivot (PR #1461)
  - PR #1470 — apr-pretrain-from-init-v1 v1.0.0 PROPOSED contract
  - PR #1471 — apr pretrain --init clap field + magic-byte validate
  - feedback_no_guessing.md — read source before forming hypothesis
  - feedback_fix_root_cause_never_route_around.md

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>