feat(stdlib): Argv — extern "env" pattern for wasm-host argv access (#86)

## Summary

- Adds `stdlib/Argv.eph` declaring three host imports (`argv_count`,
`argv_arg_len`, `argv_arg_get`) any wasm host embedding an ephapax
module must implement.
- Follows the WASI `args_get` / `args_sizes_get` buffer shape — hosts
can either bridge to WASI directly or implement the symbols natively.
- No compiler change. Existing `extern "env" { ... }` infrastructure in
`src/ephapax-wasm/src/lib.rs` (`collect_extern_imports`) handles it.

## Why

Phase 14a.1 of porting the gossamer CLI (`gossamer/cli/`) from native
Zig to typed-wasm Ephapax. The CLI needs argv access for subcommand
dispatch (`gossamer dev` vs `gossamer build` etc.). Doing it as a stdlib
`extern "env"` pattern rather than a new compiler intrinsic keeps the
compiler footprint zero while unblocking any wasm-host launcher.

Higher-level helpers (slice→String, full `Array<String>`
materialisation) are deferred until region-scoped String allocation and
a List/Array story stabilise in the v2 stdlib. Documented in the module
header.

## Test plan

Verified end-to-end on this branch:

- [x] \`ephapax parse stdlib/Argv.eph\` — 3 extern fns recognised
- [x] \`ephapax compile stdlib/Argv.eph -o /tmp/argv.wasm\` — 727-byte
.wasm
- [x] Wasm Import section inspection shows \`env::argv_count\`,
\`env::argv_arg_len\`, \`env::argv_arg_get\` emitted as proper host
imports alongside the baseline \`env::print_i32\` /
\`env::print_string\`.
- [ ] Reviewer: confirm naming aligns with future WASI-compat plan (if
any).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

stdlib/Argv.eph

@@ -0,0 +1,37 @@
+// SPDX-License-Identifier: PMPL-1.0-or-later
+// Copyright (c) 2026 Jonathan D.A. Jewell (hyperpolymath) <j.d.a.jewell@open.ac.uk>
+//
+// Ephapax.Argv — command-line argument access for wasm modules
+// compiled by ephapax.
+//
+// Argv lives at the wasm-host boundary, so it is exposed as an
+// `extern "env" { ... }` block rather than a compiler intrinsic. Any
+// host that embeds an ephapax .wasm module (e.g. the gossamer launcher,
+// a wasmtime wrapper, a browser harness) must provide implementations of
+// `env::argv_count`, `env::argv_arg_len`, and `env::argv_arg_get`.
+//
+// Buffer convention follows the WASI args_get/args_sizes_get shape:
+// the host writes raw UTF-8 bytes into a caller-supplied region of the
+// wasm linear memory. This avoids any allocation contract between host
+// and guest.
+//
+// Host contract:
+//   - argv_count() returns the number of CLI arguments, including argv[0].
+//   - argv_arg_len(idx) returns the byte length of argument `idx`, or -1
+//     when `idx` is out of range. The length excludes any trailing NUL.
+//   - argv_arg_get(idx, buf_ptr, buf_len) copies argument `idx` into the
+//     wasm linear-memory region [buf_ptr, buf_ptr + buf_len). Returns the
+//     number of bytes actually written (== argv_arg_len(idx) on success),
+//     or -1 if `idx` is out of range or `buf_len` is insufficient.
+//
+// Higher-level helpers (slice → String, full Array<String> materialisation)
+// will land once region-scoped String allocation and a List/Array story
+// stabilise in the v2 stdlib. For now, callers work at the byte level.
+
+module Argv
+
+extern "env" {
+  fn argv_count(): I32
+  fn argv_arg_len(idx: I32): I32
+  fn argv_arg_get(idx: I32, buf_ptr: I32, buf_len: I32): I32
+}