docs(release): sync roadmap, changelog, guides, and README for 1.0.0

[davydog187]

Docs sync for 1.0.0: roadmap, changelog, guides, README true-up

Plan: .agents/plans/A49-release-docs-sync.md
Plan: A49

Goal

Bring every user-facing doc and status artifact in line with reality
(20/29 official suite files passing, rc.3 → 1.0.0 pending) so the
1.0.0 cut is documentation-clean.

Changes (per scope item)

1. ROADMAP.md — Status section re-dated 2026-07-05 and updated to
   20/29 (9 fully clean + 11 with skip-ranges; 5 whole-file skips + 4
   permanent non-goals). Suite gate under "Current focus" marked MET.
2. CHANGELOG.md —
   • Unreleased: added fix(lua): hand callbacks Lua.t regardless of entry point #379 (callbacks receive Lua.t regardless of
     entry point) under Changed, fix(string.rep): allocate proportionally to result, not repeat count #376 (string.rep proportional
     allocation) under Fixed.
   • New Documentation entry: suite passes 20/29, with the 9
     exclusions listed by category (filesystem/subprocess; capability;
     perf-bound; PUC error-wording) — satisfies the A13 gate's
     "exclusions documented in CHANGELOG".
   • Normalized heading/link-ref mismatch: stripped the v prefix from
     ## [vX] headings so they resolve against the no-v link refs, and
     made ## Unreleased → ## [Unreleased] to use the defined ref.
   • Added an "Upgrading from 0.x (Luerl-based versions)" section near
     the top (with a pointer note) synthesizing the rc.0 breaking changes.
3. guides/sandboxing.md — Split the CPU/memory section: a new
   "Limiting CPU work" leads with Lua.new(max_instructions: n) as the
   in-VM CPU defense (catchable "instruction budget exceeded"), keeping
   the process/Task wrapper as the complementary wall-clock defense.
   "Putting it together" now includes :max_instructions.
4. README.md — added utf8 to the implemented-stdlib list; qualified
   the host-filesystem claim (deny-by-default; Lua.set_lua_paths/2 /
   Lua.load_file!/2 opt in); added import Lua to the Tour so pasted
   ~LUA examples compile.
5. lib/lua.ex — load_file! doc now links the Lua 5.3 manual.
6. lib/lua/runtime_exception.ex — moduledoc :source default is now
   <eval> (was <stdin>).
7. guides/mix_tasks.md — replaced the stale @ready_tests reference
   with the test/lua53_skips.exs reality, refreshed the sample output
   to a real raw run (9 passing / 17 failing / 3 timeout), documented
   --status / --audit, and noted the canonical 20/29 count. Same
   staleness fixed in the mix lua.suite moduledoc (tasks/lua.suite.ex).
8. test/lua_test.exs — removed the disjoint refute encoded == "nil"
   (redundant after assert encoded == nil) that produced a type warning.
9. Doctest wiring — added doctest Lua.Parser.Error to
   test/lua/parser/error_unit_test.exs; the example passes.
10. Plan hygiene — B17 → status: merged (noting the option shipped
    renamed to :max_instructions); A33 → status: merged (pointing at
    benchmarks/BASELINE.md).

Verification

mix format                        # clean
mix compile --warnings-as-errors  # clean
mix test                          # 2573 passed, 7 skipped, 30 excluded, 0 failures
mix docs --warnings-as-errors     # clean

The item-8 type warning is gone from the compile output; the new
Lua.Parser.Error doctest is included in the 65 doctests.

Out of scope (intentional)

• Source behavior changes (except the item-8 test-warning fix and item-9
  doctest wiring).
• API changes (sibling PR) — Unreleased entries are kept in labeled
  subsections to keep the merge conflict small.
• The 1.0.0 version bump / release CHANGELOG section (release-cut PR).
• Blog post, website.