feat: `[Finally]` section + `later` assert modifier for EXIT NEVER entries

[sleipi]

Summary

Two new features for EXIT NEVER (background process) entries:

1. [Finally] section — send a signal to the process at file-end and assert exit behavior
2. later assert modifier — defer assertion evaluation until file-end

later modifier

Asserts marked with later are not evaluated during polling. They remain open and are checked against the accumulated output at file-end (before [Finally]).

@timeout 5000
php -S localhost:8080
EXIT NEVER
[Asserts]
stderr contains "Development Server"
stderr contains later "[404]: GET / - No such file or directory"

The 404 log is produced by a later entry (e.g. curl localhost:8080). later decouples the assertion from polling timing.

[Finally] section

Entry-scoped section for EXIT NEVER entries. Executed at file-end in LIFO order (before @defer).

Syntax:

[Finally]
<SIGNAL> EXIT <code> [timeout <ms>]
<optional asserts>

Examples:

[Finally]
TERM EXIT 0 timeout 3000
stderr contains "graceful shutdown... bye"

[Finally]
KILL EXIT 137

[Finally]
TERM EXIT 0

Semantics

• SIGNAL — signal name to send (TERM, KILL, INT, HUP, QUIT, etc.)
• EXIT <code> — expected exit code (explicit, because processes may catch signals and exit cleanly)
• timeout <ms> — max wait time for process to die (default: 1000ms)
• If timeout reached → FAIL (no automatic SIGKILL escalation)
• Asserts after the signal line evaluate against the process output after signal delivery

Signals and exit codes

Signal	Unhandled exit code
TERM	143 (128+15)
KILL	137 (128+9)
INT	130 (128+2)
HUP	129 (128+1)
QUIT	131 (128+3)

Processes with signal handlers may exit with any code (e.g. EXIT 0 for graceful shutdown).

Execution order at file-end

1. All regular entries complete
2. later asserts evaluated (against accumulated output)
3. [Finally] sections executed (LIFO)
4. @defer entries executed (LIFO, cleanup fallback)

Full example

# Start web server
@timeout 5000
php -S localhost:8080
EXIT NEVER
[Asserts]
stderr contains "Development Server"
stderr contains later "[404]: GET / - No such file or directory"
[Finally]
TERM EXIT 0 timeout 3000
stderr contains "graceful shutdown... bye"

# Trigger 404 log
curl localhost:8080
EXIT 0

Design decisions

• @timeout only controls assert polling deadline (unchanged semantics)
• later ignores @timeout — it waits until file-end
• [Finally] is decoupled from @timeout — always triggers at file-end
• timeout in [Finally] is a keyword on the signal line, not a directive (directives only appear before commands)
• Default Finally timeout: 1000ms
• No automatic SIGKILL fallback on timeout — @defer handles cleanup if needed