spawn plan head: offer the plan only the user arguments, never argv[0] (#669)

[ronaldtse]

What

Both array-form call sites of the spec-30 spawn plan head (POSIX hook + windows hook) passed ruby's invoke.cmd.argv_buf to tebako_spawn_runtime_plan including its first token — the child argv[0] that rb_exec_fillarg packs. The plan composes [exe, --tebako-image…, --tebako-entry, <name>, <args…>], so the spawned runtime received the program name as its first user argument. For the java engine edge the JVM gets java as its main class:

Error: Could not find or load main class java
Caused by: java.lang.ClassNotFoundException: java

How it stayed latent

Every real-world engine spawn was shell form until now (the shell bridge takes a different code path). ruby-jing 0.1.0 switched from backticks to Open3.capture2e(*argv) — argv form, made upstream specifically for tebako-like environments (no /bin/sh reachability). The metanorma 1.17.0 payload carries it; the dogfood full-compile legs fail at the RELAX NG validation step on macOS + linux (nightly run 36230144734; the 1.16.9 payload with ruby-jing 0.0.3 shell form was green the night before).

Fix

At both call sites, advance past argv_buf's first NUL-terminated token before packing args — the plan supplies the entry name itself via --tebako-entry. A custom argv0 (the [cmd, name] array form) is likewise never a user argument. The 3.2/3.3/3.4 msys carriers get the byte-identical diff payload (parity spec enforced).

Closes the source-factory half of tamatebako/tebako#669.

Validation

• tools/apply apply-clean with the fix present in process.c, latest version of every line (3.1.7, 3.2.11, 3.3.12, 3.4.10, 4.0.7) × darwin + msys
• bundle exec rspec 116 examples, 0 failures (incl. the msys byte-parity gate)
• End-to-end proof lands with the next tebako-runtime-ruby build: the metanorma dogfood's jing leg goes green on a fixed runtime (no payload repin needed — the ABI-line resolution picks up the newest compatible runtime)

Draft rationale

Draft per the ecosystem rule: merging to main is owner-gated. The downstream chain on merge: source-factory tarball → tebako-runtime-ruby rebuild (new runtime release) → dogfood re-run.