feat: integrate sphinx-mounts for external source bundles

[ubmarco]

Summary

Alternative POC for surfacing RST/Markdown content from outside docs/
(generated under bazel-bin/, or in-repo next to its source) in the
Sphinx build without copying or symlinking. Wires the
sphinx-mounts extension into
the docs() Bazel macro: files stay where they live; the IDE reads
the same ubproject.toml Sphinx reads, so as-you-type validation
works without invoking Sphinx.

Demo: mounts src/docs/ at docs/internals/code_docs/ under the
host's "Internals" section. A cross-bundle :satisfies: link from a
host tool_req to the bundle's stkh_req__docs__mounts proves
traceability works across the mount boundary as a first-class
sphinx-needs relation — using only stock metamodel relations;
no score_metamodel/metamodel.yaml change is needed.

What's included

• mount() helper + mounts parameter on docs() (docs.bzl).
• files_to_dir Starlark rule that turns a glob into a single
  declared-directory output under bazel-bin/.
• score_mounts Sphinx extension: parses MOUNTS, resolves runfile
  paths, emits a [[mounts]] TOML fragment with portable
  confdir-relative paths pointing at the real source location
  (not the bazel-bin copy), and generates a sanitized per-bundle
  ubproject.toml at each in-repo src_root so ubCode's
  directory-walk lookup finds the project's type system from inside
  the bundle.
• New sandboxed HTML build target //:docs_html alongside
  //:needs_json.
• Comprehensive how-to at docs/how-to/mount_external_sources.rst
  (rationale, comparison with materialise-a-tree approach, full
  API reference, caveats).
• Demo bundle in src/docs/, mounted under docs/internals/code_docs/.
• Cross-bundle traceability demo: a host-side tool_req
  (tool_req__docs_mount_traceability in
  docs/internals/requirements/requirements.rst) carries a
  :satisfies: link to the bundle-side stakeholder requirement
  (stkh_req__docs__mounts in src/docs/requirements.rst).

Test plan

• bazel test //src/extensions/score_mounts:score_mounts_tests — 20 unit tests.
• bazel build //:needs_json — sandboxed needs.json build.
• bazel build //:docs_html — sandboxed HTML build.
• bazel run //:docs_check — regenerates docs/ubproject.toml and src/docs/ubproject.toml; both contain the type catalogue.
• bazel run //:docs — host index.html shows "Code Docs" auto-extended into the internals/index toctree; _build/internals/code_docs/{index,overview,requirements}.html exist.
• tool_req__docs_mount_traceability in docs/ :satisfies: stkh_req__docs__mounts in the mounted bundle without sphinx-needs warnings.

Notes for reviewers

• The trade-off matrix in docs/how-to/mount_external_sources.rst
  explains when to prefer mounts over a materialise-a-tree approach
  (the two are not mutually exclusive).
• docs/ubproject.toml is gitignored (existing convention);
  src/docs/ubproject.toml is added to .gitignore for the same
  reason.
• Pre-existing bug fixed. needs.types, needs.links, and
  needs.fields were being stripped from the generated
  ubproject.toml because needscfg_exclude_defaults = True
  filtered values matching sphinx-needs' declared defaults. Without
  the type catalogue the TOML was effectively useless to IDEs (e.g.
  ubCode could not resolve tool_req). Flipped to False in
  score_sync_toml/__init__.py. The generated file grows, but the
  values are now actually present where consumers expect them.
• No metamodel changes. score_metamodel/metamodel.yaml is
  untouched in this PR. The cross-bundle link uses tool_req's
  existing optional_links: satisfies: gd_req, stkh_req, feat_req, comp_req declaration — by authoring the mount-feature
  requirement as a stkh_req (which has no link constraints), the
  host's tool_req can satisfy it via the stock relation.

[github-actions]

License Check Results

🚀 The license check job ran with the Bazel command:

bazel run --lockfile_mode=error //src:license-check

Status: ⚠️ Needs Review

Click to expand output

[License Check Output]
Extracting Bazel installation...
Starting local Bazel server (8.6.0) and connecting to it...
INFO: Invocation ID: 178e65bc-d951-4ad8-a656-c9496879328f
Computing main repo mapping: 
Loading: 
Loading: 0 packages loaded
Loading: 0 packages loaded
Loading: 0 packages loaded
    currently loading: src
WARNING: Target pattern parsing failed.
ERROR: Skipping '//src:license-check': no such target '//src:license-check': target 'license-check' not declared in package 'src' defined by /home/runner/work/docs-as-code/docs-as-code/src/BUILD
ERROR: no such target '//src:license-check': target 'license-check' not declared in package 'src' defined by /home/runner/work/docs-as-code/docs-as-code/src/BUILD
INFO: Elapsed time: 5.995s
INFO: 0 processes.
ERROR: Build did NOT complete successfully
ERROR: Build failed. Not running target

[github-actions]

The created documentation from the pull request is available at: docu-html