docs: describe the Fixtures-based full-benchmark flow accurately

[BryanFRD]

Closes #130. Closes #131.

The action generates fixtures via the FerrLabs/Fixtures action from a JSON definitions directory, but the docs still described the old local-binary flow and mislabelled the input format:

• fix: definitions input describes TOML but definitions are JSON #131 — action.yml definitions input said "Path to TOML definitions"; the generator only reads JSON (and the value is passed straight to FerrLabs/Fixtures, whose input is JSON). Now: "Path to the directory of JSON fixture definitions…", matching Fixtures.
• docs: README full-benchmark section is stale (cargo run generate-fixtures vs FerrLabs/Fixtures action) #130 — README full section + Requirements claimed fixtures come from cargo run --release --bin generate-fixtures (a binary the action never invokes). Rewritten to the real flow: fixtures come from FerrLabs/Fixtures (caller supplies the definitions dir), and the project must build a release binary (cargo build --release) which the action puts on PATH. Dropped the generate-fixtures requirement.