Build the three-layer jlreq workspace and optimize layout pipelines

[P4suta]

Summary

• provide the dependency-free no_std + alloc composer in jlreq-core, the draw-ready jlreq facade, and the protocol-v1 conformance runner
• optimize shaping, span resolution, font fallback, construct lookup, line measurement, result queries, generated spacing tables, and bounded NDJSON transport
• split the large core, facade, and conformance pipelines into focused private modules without changing the public API
• add deterministic hot-path regression tests and a reproducible release median benchmark

Optimization evidence

A paired five-sample run against the unchanged previous head on the same machine observed:

Workload	Previous head	Optimized
reused engine, Japanese	31.96 ms	9.31 ms
bidi plus font fallback	13.66 ms	3.31 ms
many spans	11.86 ms	4.72 ms
hit/caret/selection	0.311 ms	0.147 ms
reused core composer, 10k clusters	68.84 ms	39.38 ms

The previous static-construct implementation did not finish two warmups plus one measurement within 120 seconds; the indexed implementation completed with a 69.6 ms five-sample median. Wall-clock values remain local evidence because later runs showed host-load variance. CI instead pins exact transition charging, the 10k-cluster search budget, and repeated-fallback shaping calls.

Local verification at d821b63

• cargo test --workspace --all-targets --all-features
• strict clippy with all features and with no default features
• rustfmt, rustdoc, feature matrix, release checks, package preflights, no_std thumbv7em, and wasm
• MSRV: Rust 1.88 for jlreq; Rust 1.85 for core, conformance, and xtask
• four nightly fuzz targets compile under the repository's Windows fuzz gate
• API freeze: 99 public items across three modules and all 22 typed style mappings
• generation, derivation, attestation, conformance (89 protocol cases), repository, purity, direction, placeholder, and mutation-ledger gates
• git diff --check and a clean post-commit worktree

The OCaml and Racket executables are not installed on the local Windows host; their independent conformance jobs remain part of GitHub CI.

Compatibility

• public jlreq and jlreq-core types, functions, errors, and feature structure are unchanged
• glyphs, positions, lines, source ranges, diagnostics, resource-limit errors, and protocol behavior remain exact
• no_std + alloc, declared MSRVs, and protocol-v1 NDJSON limits are preserved

Publication boundary

Merging this pull request performs no crate upload, tag, GitHub Release, or release workflow dispatch. The release workflow remains manual and protected.