feat(installer): dispatch package eventsHEADmain

InstallationManager left both PRE_PACKAGE_* and POST_PACKAGE_* as empty
stubs, so a subscriber never ran at all and the difference from upstream was
silent rather than an explicit error.

Operations cross the boundary as R-table entities with generated proxy
stubs. Materializing them the way a Link crosses is not possible: a
materialized value is revived by unserialize() on the child side, so its
properties never pass through the wire decoder and a nested handle
descriptor would not come back as a stub — and an operation always holds a
PackageInterface. execute() now shares one Rc per operation through the
whole batch pipeline, so a plugin sees one object for both the pre- and the
post-event of an operation, as it does in PHP.

POST_PACKAGE_* also moves out of the operation's promise chain into the
post-exec callback list PHP runs after waitOnPromises().

The stub generator materializes non-public class constants verbatim now,
which the operation classes need for their `protected const TYPE`: a
constant has no entity behind it, so a copy in the worker cannot diverge,
and keeping the declared visibility exposes nothing the real class hides.

The E2E fixture added here compares the recorded events against upstream
Composer. It also surfaced that upstream starts an operation's chain where
it is built (a null prepare() becomes an already-fulfilled React promise
whose handlers run through the immediately drained queue) while this port
only drives its futures in wait_on_promises, so the repository state a
pre-event observes differs; that half of the comparison is a separate
`#[ignore]`d test.

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

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