lua: os stub (time/clock/date/difftime/getenv/...) +8 tests

plans/lua-on-sx.md

@@ -72,7 +72,7 @@ Each item: implement → tests → tick box → update progress log.
 - [x] `math` — full surface
 - [x] `table` — `insert`, `remove`, `concat`, `sort`, `unpack`
 - [x] `io` — minimal stub (read/write to SX IO surface)
-- [ ] `os` — time/date subset
+- [x] `os` — time/date subset
 
 ### Phase 7 — modules + full conformance
 - [ ] `require` / `package` via SX `define-library`/`import`
@@ -82,6 +82,7 @@ Each item: implement → tests → tick box → update progress log.
 
 _Newest first. Agent appends on every commit._
 
+- 2026-04-24: lua: `os` stub — time/clock monotonic counter, difftime, date (default string / `*t` dict), getenv/remove/rename/tmpname/execute/exit stubs. Phase 6 complete. 342 tests.
 - 2026-04-24: lua: `io` stub + `print`/`tostring`/`tonumber` globals. io buffers to internal `__io-buffer` (tests drain it via `io.__buffer()`). print: tab-sep + NL. tostring respects `__tostring` metamethod. 334 tests.
 - 2026-04-24: lua: `table` lib — insert (append / at pos, shifts up), remove (last / at pos, shifts down), concat (sep, i, j), sort (insertion sort, optional cmp), unpack + table.unpack, maxn. Caught trap: local helper named `shift` collides with SX's `shift` special form → renamed to `tbl-shift-up`/`tbl-shift-down`. 322 tests.
 - 2026-04-24: lua: `math` lib — pi/huge + abs/ceil/floor/sqrt/exp/log/log10/pow/trig (sin/cos/tan/asin/acos/atan/atan2)/deg/rad/min/max (&rest)/fmod/modf/random (0/1/2 arg)/randomseed. Most ops delegate to SX primitives; log w/ base via change-of-base. 309 tests.
@@ -109,6 +110,7 @@ _Shared-file issues that need someone else to fix. Minimal repro only._
 
 ## Known limitations (own code, not shared)
 
+- **`os` library is a stub** — `os.time()` returns a monotonic counter (not Unix epoch), `os.clock()` = counter/1000, `os.date()` returns hardcoded "1970-01-01 00:00:00" or a `*t` table with fixed fields; `os.getenv` returns nil; `os.remove`/`rename` return nil+error. No real clock/filesystem access.
 - **`io` library is a stub** — `io.write`/`print` append to an internal `__io-buffer` (accessible via `io.__buffer()` which returns + clears it) instead of real stdout. `io.read`/`open`/`lines` return nil. Suitable for tests that inspect output; no actual stdio.
 - **`string.find`/`match`/`gmatch`/`gsub` patterns are LITERAL only** — no `%d`/`%a`/`.`/`*`/`+`/etc. Implementing Lua patterns is a separate work item; literal search covers the common case.
 - **`string.format`** supports only `%s`, `%d`, `%f`, `%%`. No width/precision flags (`%.2f`, `%5d`).