Accept bare IDENT in kind-qualified brackets (#119)

[iskandr]

Summary

Closes #119 (step 1 only — step 2 labels deferred until there's real demand).

The string DSL now accepts bare identifiers inside kind-qualified brackets in addition to the quoted form:

```
affinity[netmhcpan] <= 500 # new
affinity[netmhcpan, "4.1b"] <= 500 # version keeps quotes (not an IDENT)
affinity['netmhcpan'] <= 500 # still parses
affinity['netmhcpan', '4.1b'].rank <= 2.0 # still parses
```

Motivated by YAML-embedded configs (e.g. vaxrank scoring YAML), where `"affinity[netmhcpan] <= 500"` is much easier to read than `"affinity['netmhcpan'] <= 500"` with nested quote escaping.

Changes

• `_Tokenizer.expect_string_or_ident()` — bracket-arg primitive that accepts either a STRING or IDENT token and returns it. Wraps the exact shape of `expect()`.
• `_postfix` (the chained `[...]` path) and `_kind_accessor` (the first-bracket path) now call the new helper at all four bracket-arg sites.
• Grammar comment updated: `BRACKET_ARG := STRING | IDENT`.
• `to_expr_string()` is unchanged — it still emits single-quoted canonical form, so round-trips and serialization are deterministic. The bare form is a read-only input sugar.

Back-compat

Fully additive. Every previously-valid expression still parses. Error surface for genuinely invalid bracket args is clearer: `parse("affinity[netmhcpan, 4]")` → `ValueError("Expected STRING or IDENT ...")` instead of the old `Expected STRING ...`.

Deferred (Step 2 of the issue)

Prometheus-style labels (`affinity[predictor=netmhcpan, version="4.1b"]`) are left out of this PR. They're the right escape hatch if positional order starts tripping users up, but Step 1 solves the immediate YAML ergonomics pain.

Test plan

• `affinity[netmhcpan].score` parses to `Field(method="netmhcpan")`; AST-equal to the quoted form
• `affinity[netmhcpan] <= 500` parses as a Comparison with qualified LHS
• `affinity[netmhcpan, "4.1b"]` carries method + version through
• Mixed forms in one bracket (`['netmhcpan', v4]`) work
• Bare form composes with `.norm(...)` transforms
• `to_expr_string()` → re-parse round-trip is AST-equal
• `affinity[netmhcpan, 4]` (number inside bracket) raises `STRING or IDENT`
• Full suite: `pytest tests/` → 1187 passed, 3 skipped

[coveralls]

coverage: 88.837% (+0.2%) from 88.589% — bare-ident-brackets into master