test(suite): document deliberate non-goals in lua53 suite

[davydog187]

Triage cluster — sandbox-refusal suite files

Plan: .agents/plans/A20-triage-sandbox-refusals.md

Goal

Decide, per file, whether each of main.lua, verybig.lua, files.lua,
attrib.lua is a permanent non-goal (skip with explanatory tag) or whether
the suite runner should be configured to stub os.execute / os.tmpname /
os.getenv so the file completes. Produce per-file follow-up plans or skip
annotations.

Decision: all four are skip-permanently. Each exercises capabilities
that conflict with this library's role as a sandboxed embedded Lua VM.

Per-file decisions

File	Failure	Decision	Reason
main.lua	os.execute() is sandboxed	skip-permanently	Tests the standalone Lua interpreter binary: shells out, writes Lua programs to temp files, invokes lua as a subprocess. We are an embedded VM with no shell-out.
files.lua	os.getenv(_) is sandboxed	skip-permanently	Tests filesystem I/O end-to-end. io.* is a stub by design.
attrib.lua	module 'io' not found	skip-permanently	Tests require semantics that depend on writing files to disk and dynamically loading them.
verybig.lua	os.tmpname() is sandboxed	skip-permanently	Tests RK opcodes / >64K constants via a os.tmpname() + io.output() + dofile() harness. The VM behaviour is interesting; the harness is not. A future plan could stub these for the suite runner only — not in scope here.

Success criteria

• Each of the four files has a documented decision. All four are
  skip-permanently; reasons live in test/lua53_suite_test.exs
@deferred_permanent map.
• test/lua53_suite_test.exs gains a comment alongside the
  @tag :skip for each of the four, and this is reflected in
  CHANGELOG.md (Unreleased) and ROADMAP.md (Deferred section).
• No fix-now follow-up plans needed. (See Discoveries — verybig.lua
  could justify a future stub-for-suite plan, but it is recorded as a
  possibility, not a commitment.)
• No regression. mix test reports 1585 tests, 0 failures, 31 skipped
  (same as pre-flight). Suite test reports 29 tests, 0 failures, 24
  skipped (5 ready / 4 deferred-permanent / 20 missing-feature).

Changes

 .agents/plans/A20-triage-sandbox-refusals.md |  2 +-
 CHANGELOG.md                                 |  9 +++++
 ROADMAP.md                                   | 22 ++++++++++-
 test/lua53_suite_test.exs                    | 57 +++++++++++++++++++++++++---
 4 files changed, 82 insertions(+), 8 deletions(-)

Key change in test/lua53_suite_test.exs: split the single @skipped_tests
list into @skipped_tests (missing features, future plans welcome) and
@deferred_permanent (a %{file => reason} map for deliberate non-goals).
The deferred-permanent tests get their own describe block with the reason
embedded in the test name and a @tag :deferred_permanent for filtering.

Discoveries

• The plan's premise that all four files fail with os.execute() /
  os.tmpname() / os.getenv(_) was correct for three of them.
  attrib.lua is a different sandbox policy: it dies on require"io"
  because there is no io global to cache. The cluster framing still
  holds — all four exercise capabilities that conflict with our sandbox
  role — but the failure mechanism for attrib.lua is package/loader-
  driven, not os.*-driven.
• verybig.lua is the most interesting of the four. The first ~140 lines
  test genuinely useful VM behaviour (RK opcodes, >64K constants), but
  they live inside a generated Lua program written to a temp file and
  executed with dofile(). A future plan could stub these primitives in
  the suite runner only to unlock that coverage. Recorded as a
  possibility, not a commitment.
• README has no "supported coverage" section. Adding one would duplicate
  what's already in ROADMAP.md's "Deferred" list. Updated ROADMAP and
  CHANGELOG instead.

Verification

mix format
mix compile --warnings-as-errors
mix test

52 doctests, 51 properties, 1585 tests, 0 failures, 31 skipped

Out of scope (intentional)

• Implementing os.execute etc. for general consumption.
• Changing the public sandbox API.
• Stubbing os.tmpname/io.output/dofile for the suite runner —
  recorded as a possible future plan, not shipped here.
• Suite files outside this cluster.