#131)

Closes the 3-way parity defect surfaced by task #126 at commit 29607ed.
The MODEL-2 tokenizer artifact (trained in task #118) has 50_257 entries
per GPT-2 industry convention (50_000 BPE merges + 256 byte-level
fallback tokens + 1 sentinel). Both pre-existing contracts pinned
50_000 exactly, and Llama370MConfig::VOCAB_SIZE was hardcoded 50_000 —
a mismatch that GATE-ARCH-370M-011 (PR #961) now catches pre-flight
but previously let through silently via the N-09 OOB escape guard in
Embedding::forward.

Option A (this PR) honors the real-world artifact and burns no
tokenizer re-training compute. Option B (retrain tokenizer at 50_000)
was rejected as muda.

Changes are byte-in-lockstep across three artifacts:
- contracts/tokenizer-bpe-v1.yaml 1.1.0 → 1.2.0:
    vocab_size 50000 → 50257
    merge_rules_count_expected 49996 → 49997
    INV-BPE-004 / GATE-BPE-004 window [49992, 50000] → [49993, 50001]
- contracts/model-families/llama-370m-sovereign-v1.yaml 1.4.0 → 1.5.0:
    architecture.vocab_size 50000 → 50257
    INV-ARCH-370M-006 desc + falsifier updated
    INV-ARCH-370M-009 shape example [50000, 1024] → [50257, 1024]
- crates/aprender-train/src/models/llama_370m.rs:
    pub const VOCAB_SIZE: usize = 50_000 → 50_257
    validate() const-fn assert updated
    falsify_gate_arch_370m_011_helper_rejects_mismatch counter-example
    switched to VOCAB_SIZE - 1 so the test exercises a real mismatch
- crates/apr-cli/src/commands/pretrain.rs:
    preflight_rejects_tokenizer_vocab_mismatch counter-example updated
    in lockstep; uses Llama370MConfig::VOCAB_SIZE.to_string() assertions
    so the test no longer rots on future vocab changes
- crates/apr-cli/src/tokenize_commands.rs:
    `apr tokenize train --vocab-size` default 50000 → 50257 (GPT-2 canon)

Param count: INV-ARCH-370M-001 band [366M, 374M] still holds — delta
is (257 × 1024) = 263_168 = 0.07%.

Validation:
- `pv validate` both contracts: 0 errors
- `cargo test -p aprender-train --lib models::llama_370m`: 9/9 pass
- `cargo test -p apr-cli --lib commands::pretrain`: 11/11 pass
- `cargo fmt --check` (touched files): clean

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>