S0308-submodule-agent-doc-budgets: split the README into documentation/

[NSchatz]

What

README.md measured 79471 bytes against the 22400-byte ceiling the umbrella budgets for a submodule README. Eight reference sections move into a new documentation/ directory, whole and byte for byte under their existing headings, each linked from the README at the point it left. README.md now measures 16441 bytes.

file	what moved
documentation/access-patterns.md	named helpers, dot-paths, structural traversal
documentation/cookbook.md	the eight message recipes
documentation/cookbook-profiles.md	the five profile recipes
documentation/cookbook-datetime.md	datetime precision and timezone fidelity
documentation/cookbook-messages.md	framing, batches, streams, type detection, structure and profile validation, pretty-print
documentation/profiles.md	the whole profile reference
documentation/real-world-tolerance.md	the four-tier model
documentation/error-handling.md	the three error types

docs-content/ was not used for this: it is the bundle pnpm pack:docs ships to the docs site, every page in it needs frontmatter and a sidebars.json entry, and check-docs.ts allows only ./<basename>.md links between pages. Putting README overflow there would change a published artifact. documentation/ is in neither the npm tarball nor the docs bundle.

No text was lost

• Heading reachability. Every heading in the README at the base commit is still reachable from README.md or from a file it links: the set difference prints nothing.
• Link resolution. Every markdown link target in README.md resolves, none is absolute and none leaves the repo root.
• Relative links inside moved text are re-based by exactly one directory, and the three README links whose target heading moved now name the file that holds it. No heading was renamed.

The public-surface gate follows the text

documentation/ joins SURFACE_PATHS in scripts/check-no-internal-refs.sh. This text was inside that gate's surface while it was README.md, and splitting it into files of its own must not quietly take it out. Checked both ways: the gate reds naming ./documentation/access-patterns.md on a seeded internal identifier, and passes on the tree as it stands (37 public-surface files scanned, up from 29).

Evidence

• pnpm check:no-emdash OK
• pnpm check:no-internal-refs OK (37 public-surface files, 121 source files)
• pnpm check:docs OK
• pnpm format:check OK
• pnpm test 152 files, 3589 passed, 28 skipped, 12 todo
• npm pack --dry-run lists CHANGELOG.md, LICENSE, README.md, TRADEMARKS.md, package.json: the same file set as before, since package.json files is unchanged and documentation/ matches no entry in it. Nothing under src/, test/, docs-content/, tsconfig.json or package.json is touched, so the docs bundle and the source bundle are unchanged too.

No parse, mapping, fixture or log line changes. Markdown and one gate's scan surface only.

Spec: pipeline/active/S0308-submodule-agent-doc-budgets/children/hl7.md.