test: add reason strings to the bare #[ignore] attributes

## Summary

Twenty `#[ignore]` attributes carry no reason string in the attribute itself, against 335 in the repo that do (`#[ignore = "..."]`). In every case the reason text is already adjacent in the file: a `MODEL_DIR` const, an `eprintln!` skip message, or a trailing comment.

## Background

A bare `#[ignore]` gives no hint in `cargo test -- --list` about why a test is skipped, so a reader cannot tell a hardware gate from a known-broken test. The convention across the repo is the `= "..."` form. This is mechanical: copy the adjacent reason into the attribute.

## Proposed Solution

Fill each bare `#[ignore]` with the reason already sitting next to it. No hardware needed; compilation (or `cargo test -- --list`) verifies.

## Implementation Notes

Verified bare sites in the current tree (20 total):

- `tests/deepseek_v4_real_model.rs:55,111,145,236,362`
- `tests/qwen38_mtp_chain_parity.rs:127,202,447,523,612,678`
- `tests/ernie4_5_moe_vl_parity.rs:132`
- `tests/qwen3_omni_moe_parity.rs:66`
- `src/models/diffusion_gemma/tests.rs:448,803`
- `src/downloader/tests.rs:547`
- `src/tokenizer/tiktoken.rs:368,383,394,405` (these carry a `// Requires model files` trailing comment but no reason in the attribute)

Note: the two `src/models/diffusion_gemma/tests.rs` sites were missed by the original enumeration; they are included above.

Convention examples: `tests/turbo_kv_e2e.rs:598`, `:633`, `:657`. Adjacent reason sources: `tests/deepseek_v4_real_model.rs:52` (`MODEL_DIR`) and its skip message; `src/tokenizer/tiktoken.rs:368` (`// Requires model files`); `src/downloader/tests.rs` skip block.

## Acceptance Criteria

- [ ] Zero bare `#[ignore]` remains under `src/` and `tests/`.

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## Original Suggestion

### Title: test: add reason strings to the twenty bare #[ignore] attributes

Twenty `#[ignore]` attributes carry no reason string, against 330 in the repo that do (`#[ignore = "..."]`). In every bare case the reason text is already sitting adjacent in the file — a `MODEL_DIR` const, an `eprintln!` skip message, or a trailing comment.

## Evidence

Bare sites: `tests/deepseek_v4_real_model.rs:55,111,145,236,362`; `tests/qwen38_mtp_chain_parity.rs:127,202,447,523,612,678`; `tests/ernie4_5_moe_vl_parity.rs:132`; `tests/qwen3_omni_moe_parity.rs:66`; `src/tokenizer/tiktoken.rs:368,383,394,405`; `src/downloader/tests.rs:547`.

Convention examples: `tests/turbo_kv_e2e.rs:598`, `:633`, `:657`. Adjacent reason sources: `tests/deepseek_v4_real_model.rs:52` (`MODEL_DIR`) and `:59-61` (skip message); `src/tokenizer/tiktoken.rs:368` (`// Requires model files`); `src/downloader/tests.rs:544-545`.

## Suggested fix

Copy each adjacent reason into the attribute. No hardware needed; `cargo test -- --list` (or just compilation) verifies.

## Acceptance criteria

- [ ] Zero bare `#[ignore]` remains under `src/` and `tests/`

Start with the 20 locations listed in the issue, especially the bare attributes in the named files under tests/ and src/. Read each adjacent MODEL_DIR, skip message, or trailing comment, then run `cargo test -- --list` or compile the project. Done means every bare `#[ignore]` under src/ and tests/ has an appropriate reason string.

Written by the indexing model from the issue text.