feat(rules): convert a rule-content-v1 statement into the public model

[e54-bot]

Fixes #531

Adds rules::statement::statement_from_content(catalog, node), the input direction of rule-content-v1. It converts one condition ({"value": node}) or action ({"call", "args"}) list item, with optional "disabled": true, into a Statement. A node from Rule::content is valid input unchanged. The format id and schema are unchanged.

StatementContentError has distinct variants, each naming the node by JSON pointer: UnknownId, ArgumentCount, InternalSentinel, Unrepresentable, Spans. Argument counts follow the same rule canonical validation applies. An ambiguous enum node and an opaque rawWorkshopAction are refused as Unrepresentable.

Verification:

• content(statement_from_content(content(s))) == content(s) over the practice-range exports and the corpus fixtures. The practice-range files are parsed in en-US and zh-CN, and the program is emitted and re-parsed in every locale that can render it (emission skips locales missing a spelling; en-US and zh-CN are asserted). Round-trip is verified only for en-US and zh-CN because the catalog lacks spellings in the other 14 locales; statement content is locale-independent.
• Refusal tests for each error variant, ambiguous enums, nested spans and disabled modifiers.
• fmt, clippy -D warnings, full tests, workshop-catalog-gen check on stable and 1.88.0.
• cargo semver-checks reports two failures (SettingsListElement.variant, LookupMatch::Builtin.description). Both are in code this PR does not touch, and no failure names the new module.

Argument-count checking is one shared CatalogEntry::rejected_arity used by canonical validation and by conversion, with an agreement test over a folded player-variable pair, a variadic entry and an undeclared-signature entry.

Not in scope: emission and edits (#532).

[Teakowa]

One defect against the repo's contract rules; everything else I checked holds.

Arity check reimplements the established path instead of sharing it (crates/workshop-rs/src/rules/statement.rs, Converter::check_arity). It copies the rule in values::validate::validate_call_signature (undeclared-signature skip, folds_player_variable_pair +1, required_param_count..param_count, variadic). workshop-rs/AGENTS.md "Sibling paths share checks" requires reusing the established path's checks and adding a test that both paths agree on the same inputs. The PR body states the rule is the same, but nothing enforces it: when validation changes (for example a new fold case), conversion silently diverges and accepts or refuses differently from canonical validation. No agreement test exists.

Required correction, smallest level: extract the count rule into one CatalogEntry helper taking the supplied count (and the fold flag), call it from both validate_call_signature and check_arity, and add one test that feeds the same call (including a folded player-variable pair, a variadic and an undeclared-signature entry) through conversion and canonical validation and asserts both accept or both refuse.

Checked and fine: fmt, clippy -D warnings, full workspace tests and catalog check pass locally; CI is green (stable, 1.88.0, public API, catalog, real-project). Round-trip passes over practice-range (en-US, zh-CN) and corpus fixtures; 5 distinct error variants with JSON pointers; spans (top-level and nested) rejected; ambiguous enum and raw action refused; additive API; docs updated; no emission or edit code (#532).

[Teakowa]

LGTM: the argument-count rule now lives once in CatalogEntry::rejected_arity and both validate_call_signature and statement_from_content call it. The new agreement test checks accept and reject for every count on a folded player-variable pair, a variadic entry and an undeclared-signature entry. fmt, clippy -D warnings and the full workspace tests pass in a fresh worktree on 99d1ec4, scope is unchanged, and all 5 CI checks are green.