t27o: Zig front end, with a round-trip check over our own corpus via gen-zig #8994

Part of the t27o service epic (linked from it). A Zig front end for t27o (lexer, parser and normalisation into the shared middle; see #8966). It goes first because t27 is closest to Zig, and trinity holds hundreds of hand-written .zig files that bees used to port by hand.

Free oracle and corpus: for every ledger spec, .t27 -> t27c gen-zig -> t27o -> .t27' must be behaviourally equivalent: the same test results under the reference and t27b. This tests t27o and also catches generator defects (it would have found #6549 and the CSE hoist, see #8197).

Acceptance: the round-trip pass rate is reported over the 1368 ledger specs, and the trinity .zig coverage table follows the format of #8970.

User Scenarios

• Given a ledger spec spec.t27, when t27c gen-zig produces Zig code and t27o parses it back to .t27', then the test results under the reference interpreter and t27b are identical.
• Given the 1368 ledger specs in the corpus, when the round-trip pipeline runs for each, then the pass rate is reported and matches the coverage table format of t27o (Ouroboros): Rust front end MVP -- skeleton, normalisation and a rustc oracle, written in t27 (Closes #8966) #8970.
• Given a Trinity hand-written .zig file, when processed through the new Zig front-end into the shared middle, then normalization succeeds without parser errors.

Requirements

• FR-001: The Zig front-end (lexer, parser, normalizer) MUST integrate into the t27o shared middle as specified in t27o (Ouroboros): Rust front end MVP -- skeleton, normalisation and a rustc oracle, written in t27 #8966.
• FR-002: The round-trip .t27 -> t27c gen-zig -> t27o -> .t27' MUST produce behaviorally equivalent specs for all 1368 ledger specs.
• FR-003: The test harness MUST execute both reference and t27b runners on each round-tripped spec and compare results.
• FR-004: The coverage table for Trinity .zig files MUST follow the format established in t27o (Ouroboros): Rust front end MVP -- skeleton, normalisation and a rustc oracle, written in t27 (Closes #8966) #8970.

Success Criteria

• t27c test roundtrip --corpus ledger exits 0 and prints "1368/1368 passed" (or pass rate percentage)
• t27c test roundtrip --corpus trinity exits 0 and emits a coverage table matching the t27o (Ouroboros): Rust front end MVP -- skeleton, normalisation and a rustc oracle, written in t27 (Closes #8966) #8970 format
• cli/t27b/src/front.rs compiles without errors after integration changes

Boundary

• cli/t27b/src/front.rs