Fix substitution keys with characters invalid in a Jinja2 identifier

[agu2347]

Fixes #1007.

render_substitution() builds a Jinja2 expression by directly interpolating the raw substitution content into a template string: env.from_string(f"{{{{{token.content}}}}}"). For a key like foo-bar, this produces the template "{{foo-bar}}", which Jinja2 parses as the expression foo - bar (subtraction of two separate names) rather than as a lookup of a single variable named foo-bar. Since neither foo nor bar exist in the substitution context, this raised UndefinedError: 'foo' is undefined, matching the error in the issue -- notice it never even mentions the -bar part, because Jinja evaluates left-to-right and raises on the first undefined name.

MyST substitutions deliberately support full Jinja2 expression syntax (filters, dotted attribute access, etc. -- confirmed by the existing circular-reference detection walking the full parsed AST for all Name nodes, not just a single key), so a blanket fix escaping all non-identifier-safe characters isn't viable without breaking that.

Fix: when the substitution's content is exactly one key already present in the variable context, look it up directly via dict subscript, bypassing Jinja2 expression parsing entirely for that case. Anything else -- arbitrary expressions, filters, dotted access, keys that aren't a literal match -- still goes through the existing Jinja2 rendering path, completely unaffected. The special env name is excluded from the fast path's reference-tracking, matching the existing exclusion in the general path.

Verification: verified end-to-end with a real Sphinx build (a shallow test-fixture based reproduction wasn't enough on its own, so I also built a throwaway Sphinx project on disk mirroring the issue's exact repro): before the fix, {{foo-bar}} renders empty with the exact 'foo' is undefined warning from the issue; after the fix, it renders correctly. Also verified normal plain keys, Jinja2 filter expressions (e.g. {{ value | upper }}), and circular-reference detection all continue to work exactly as before.

Testing: added an integration test (test_substitutions_dashed_key) with a new sourcedirs fixture covering: a dashed key, a plain key, and a circular reference (to confirm circular-reference detection still triggers correctly for a dashed key too, exercising the fast path's reference tracking). I confirmed the test fails with the original code (produces the exact reported UndefinedError and leaves the dashed key unrendered) and passes with the fix.

Ran the full existing test suite: 1241 passed (1240 baseline + 1 new), 12 skipped. The 4 remaining failures (missing linkify-it-py optional dependency, and pygments/docutils version-related rendering diffs) are present identically on a clean checkout of master, unrelated to this change.