feat(signal): abort on SIGINT, SIGTERM and SIGHUP at checkpoints

The SignalHandler port was a no-op stub, so all four of Composer's abort
paths were dead code: nothing removed a half-created project, reverted
composer.json, or cleaned up half-installed packages.

Composer runs those handlers from pcntl callbacks, which a Rust signal
handler cannot do -- it may touch nothing beyond atomics. SignalSubscription
records the signal instead, and the abort runs from checkpoints on the normal
call stack, where the clean-up can borrow the state it needs. That also
resolves the closure-capture TODO(phase-c)s in RequireCommand and
InstallationManager, and replaces exit_with_last_signal's exit(0) with the
restore-and-re-raise Seld\Signal does.

A subscription is live only inside the four abort regions, so elsewhere the
signals keep their default disposition and kill the process at once. It is
installed without SA_RESTART so a signal interrupts an interactive prompt
rather than resuming the read. A signal reaches only the innermost
subscription, reproducing SignalHandler's single-stack dispatch.

Drop SignalRegistry, SignalableCommandInterface and the Application wiring
for them: nothing in Composer reaches that path, and SignalHandler discards
whatever they register. Signal handling from plugins and scripts is
undefined behavior; see docs/dev/signals.md.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

2 files changed, 56 insertions, 0 deletions

diff --git a/docs/dev/signals.md b/docs/dev/signals.md
new file mode 100644
index 00000000..9616f0ee
--- /dev/null
+++ b/docs/dev/signals.md
@@ -0,0 +1,46 @@
+# Signals
+
+## In Composer core
+
+Composer has three routes for handling signals:
+
+* `Seld\Signal\SignalHandler`
+* `Symfony\Component\Console\SignalRegistry\SignalRegistry` and `SignalableCommandInterface`
+* PHP builtins (`pcntl_signal()`, etc.)
+
+Of these, upstream Composer itself only ever uses `SignalHandler`, and Shirabe
+behaves roughly the same way. The difference is when the interruption takes
+effect, as described in [known incompatibilities](../known-incompatibilities.md).
+
+> Composer runs its abort handler almost immediately after the signal arrives.
+> Shirabe, however, runs it at the next checkpoint instead, so stopping `shirabe`
+> command by `Ctrl+C` may take more time than Composer.
+
+In PHP, the VM checks for a pending signal at the end of a loop and on a
+function call, which gets the signal handler run almost immediately after the
+signal arrives. Rust has no such mechanism, so we place the checkpoints by
+hand; grep for `signals.is_triggered()` to find them. They are not as
+fine-grained as a function call, so more work runs between receiving the signal
+and starting the abort than Composer would let through, and it takes longer.
+
+## In plugins and scripts
+
+Shirabe treats signal handling in plugins and scripts as undefined behavior,
+for two broad reasons.
+
+The first is that Shirabe consists of a Rust core process and a PHP worker
+process that runs the plugins and scripts, which makes faithful reproduction
+difficult.
+
+The second is that Composer itself does not fully account for plugins and
+scripts subscribing to signals either. The `SignalHandler` that Composer uses
+to handle signals overwrites an existing signal handler unconditionally. So
+even when a plugin or script installs a handler through Symfony Console or a
+PHP builtin, that handler is lost the moment execution reaches a place where
+Composer uses `SignalHandler`.
+
+For these two reasons, Shirabe today neither restricts plugins and scripts from
+installing signal handlers nor does anything special about it. What happens
+when they do is not guaranteed.
+
+This stance may be withdrawn if a legitimate use case turns up.
diff --git a/docs/known-incompatibilities.md b/docs/known-incompatibilities.md
index daaa8bcd..97e4dad7 100644
--- a/docs/known-incompatibilities.md
+++ b/docs/known-incompatibilities.md
@@ -39,6 +39,16 @@ behavior is slightly different. See [docs/dev/xdebug.md](./dev/xdebug.md)
 for details.
 
 
+## Signals
+
+Composer runs its abort handler almost immediately after the signal arrives.
+Shirabe, however, runs it at the next checkpoint instead, so stopping `shirabe`
+command by `Ctrl+C` may take more time than Composer.
+
+Signal handling in plugins and scripts is undefined behavior: it may or may not
+work. See [docs/dev/signals.md](./dev/signals.md) for details.
+
+
 ## Plugins
 
 ### Reflection