build: reorganize repo layout (docs_src, dist/meson, test/bench, LSP compdb) - #128

The scraped Oracle DocBook under docs/ is superseded by docs_src/ (its
content was migrated at ~100% retention, verified in PRs #119/#120). The
ENTIRE old docs/ tree is also preserved on the gh-pages branch under
docs/db/5.3.28/ (confirmed: gsg C/CXX/JAVA variants and the 528-file java/
Javadoc are all present there), so removing it from master loses nothing
permanently.

Disposition of each docs/ subtree:
- api_reference, articles, bdb-sql, collections, gsg (C), gsg_db_rep (C),
  gsg_txn (C), installation, porting, programmer_reference, upgrading,
  index.html: MIGRATED into docs_src/ -> removed.
- java (528 Javadoc HTML), and the CXX/JAVA variants of the gsg guides:
  NOT yet in docs_src (Java is a supported binding). DEFERRED to a follow-up
  docs PR; the content is safe on gh-pages (docs/db/5.3.28/) meanwhile. Removed
  from master here only because the trees are interleaved with migrated ones.
- ssi/ (M2/M4 SIREAD design notes): internal design intent, not user docs ->
  RELOCATED to rfc/0003/ as the working notes of a new RFC 0003 (SSI, Status
  Implemented), with an INDEX.md row.
- legal + license: PRESERVED. docs/license/license_db.html and the
  per-component license texts (ASM/BSD/CDDL/HARVARD/SPL) relocated to a
  top-level LICENSES/ (with a README) -- these are the fork's legal basis and
  are not deleted. No build/source referenced docs/legal or docs/license.

Build targets (dist/Makefile.in):
- install_docs now installs docs-build/html if present (build is a separate
  `make docs` concern; a plain install never needs the docs toolchain).
- `make docs`       -> python3 docs_src/build.py (HTML+man, +PDF if weasyprint)
- `make docs-check` -> the docs.yml gates locally (build+self-check, API
  completeness, spelling, man-lint)
- `make bench`      -> build the test/bench microbenchmark drivers (CC=cc, to
  bypass the parent build's libtool CC wrapper)
- `make compdb`     -> compile_commands.json via bear
  All guard on their toolchain (python3/pandoc/bear) and skip-with-note if
  absent, so they never hard-fail a build that isn't about them.

docs.yml: the no-loss gate (verify_all.py, which diffed the old docs/ against
docs_src/) is retired -- its migration source no longer exists on master; the
surviving hard gates (build, completeness, spelling, links, man-lint) stay.

lab/bench is a real microbenchmark regression suite (scale_bench, lock_bench,
ssi_abort_bench, tproc_b/c/h) -- move it under test/ where the rest of the
regression tooling lives. Its Makefile's `BDB ?= ../../build_unix` default
still resolves from the new location (same directory depth), so it is
unchanged; `make bench` (Autoconf) and `ninja bench` (Meson) now build the
drivers via the targets added in the previous commit.

With bench moved and the LSM prototype already graduated to rfc/0001/ (PR
#127), lab/ held only README.md, whose content was entirely bench docs -- its
useful parts move to test/bench/README.md and lab/ is removed. Fixes the two
lab/bench/scale_bench references in rfc/0002.

Move the Meson build logic (meson.build) and its helpers (meson/gen_header.py,
meson/db_subs.json) under dist/, alongside the Autoconf build files.

The root-meson.build problem: Meson requires meson.build AND meson_options.txt
at the setup directory, and its subdir() cannot climb out with '..'. So:
- dist/meson.build holds the real build logic, anchored at the project root via
  `root = '..'` (sources under src/, templates, dist/clib_port.in all resolve).
- A thin root meson.build owns project()/cc/options and does subdir('dist');
  because subdir() shares the interpreter scope, dist/meson.build's vars (inc,
  db_h, thread_dep, libdb) stay visible to the shim, which then enters
  test/pbt (the PBT subdir must run from the root, not dist/, since subdir
  cannot use '..').
- meson_options.txt stays at the repo root: Meson binds the options file to the
  project() directory. Moving it to dist/ would force `meson setup build dist`,
  breaking the zero-arg `meson setup build` and the nix flake's `src = ./.`
  meson hook. This is the one file the relocation can't move cleanly.

The library now builds under <builddir>/dist/libdb.so; consumers updated:
- ci.yml smoke test: -I/-L build-meson/dist, LD_LIBRARY_PATH=build-meson/dist
- ci-extended.yml config-parity: build-meson/dist/db_config.h
- dist/android/build_android.sh: copy <builddir>/dist/libdb.so up to the stable
  <builddir>/libdb.so artifact path
- android.yml / pbt.yml path filters: dist/meson.build, dist/meson/**
- ocr/rule.json: dist/meson.build / dist/meson/

Also adds Meson run-targets (dist/meson/run_docs.py, run_bench.py):
  ninja docs / docs-check   render docs_src/ + run the gates
  ninja bench               build the test/bench drivers vs the fresh libdb

README: split Building into Autoconf + Meson sections with the new targets.

Verified: meson setup build && ninja (libdb.so), ninja docs/bench, nix build
.#libdb-meson -> result/lib/libdb.so, meson setup -Dhegel=enabled + ninja (PBT).

The Autoconf build emits no compilation database, so `make compdb` (added
earlier) wraps the build with bear to produce a repo-root
compile_commands.json for clangd. Add bear to the nix dev shell so the target
works out of the box. (The Meson build emits build/compile_commands.json
automatically via Ninja; both paths are documented in the README and
git-ignored.)