docs: architecture and README drift from the implementation

[OmarAlJarrah]

Problem

Several places where the design docs and README no longer match the code they specify — each small, but these documents are declared normative, so drift is costly:

1. Package layouts omit four real packages. docs/architecture.md §3 and README's layout table list only ir/ (+irtest), compilers/*, pass/, emitters/*, engine/, cmd/morphic/. Missing: cmd/morphic-harness/, internal/harness/, internal/archtest/, ir/irverify/. The code is well-placed (the archtest comment explains why internal/harness sits outside the layering) — the docs are what's stale. README's table also says cmd/morphic imports "engine", but it imports ir too.
2. §2.1 still shows the pre-rename contract. architecture.md §2.1 shows Parse(ctx, sources, Options); the method is Compile since the frontend→compiler rename (refactor(compilers)!: rename frontend→compiler and backend→emitter #5). engine/engine.go also retains front/fronts identifiers and an "engine: parse %q" error prefix — worth renaming while touching it.
3. emitter-design.md §2.1 contradicts the implemented registry philosophy. It prescribes a registry "populated by each target's init() — mirroring the compiler registry", but the actual compiler registry explicitly forbids init-time self-registration (compilers/compilers.go:46-47) and CLAUDE.md bans mutable globals. If uncorrected, Milestone 3 gets built against the wrong pattern citing "mirroring" as authority.
4. architecture.md §3 lists a future compilers/swagger/ directory whose job ("2.0 lift → openapi compiler") would require the sibling import the layering rules forbid — while Milestone 2's text says the lift lives inside the OpenAPI compiler. Drop the directory from the layout.
5. README documents one diagnostic format; the CLI prints three (path#pointer, bare IR ID, bare line:col). Document the fallback forms.

Proposed direction

One docs PR covering all five, plus the engine identifier rename if desired.

Acceptance

• Layout listings match find . -type d; the compiler contract shown matches compilers.Compiler; emitter-design's registration section matches the engine-composed pattern; README's diagnostic-format section matches actual output.

[OmarAlJarrah]

Items 1 and 4 are resolved by #227.

Item 4 — compilers/swagger/ is dropped from architecture.md §3, with a line saying why there is no such directory: the lift normalizes to the OpenAPI 3.x shape and lives inside that compiler, so a sibling package calling into compilers/openapi is the one import the layering rules in that same section forbid.

Item 1 — both layout listings now match the tree. architecture.md §3 was rewritten for the compiler split and includes cmd/morphic-harness/, internal/harness/, internal/archtest/, internal/testspec/ and ir/irverify/; README's table gains the same, and its cmd/morphic row says engine + ir rather than engine alone. Both now point at internal/archtest's rules map as the source of truth and give the command that derives the layout, so the next drift is a failing test rather than a stale table.

Still open here: item 2 (§2.1 shows Parse where the method is Compile; engine retains front/fronts identifiers and an "engine: parse %q" prefix), item 3 (emitter-design.md §2.1 prescribes an init()-populated registry, which the compiler registry explicitly forbids), and item 5 (README documents one diagnostic format; the CLI prints three).