bundler: route every cross-thread event to the bundle thread through one post() - #37997

Two doc comments in this file went stale with this PR: (1) the `bv2_mut` caller list at line 1413 still names `…__addError`, but `JSBundlerPlugin__addError` now calls `post::<Plugin…Settled>` directly and never reaches `bv2_mut`; (2) the NOTE at lines 1396-1398 still says Resolve/Load live in bun_bundler "so that BundleV2 can operate on them directly (`on_resolve_async`/`on_load_async`)" — both of those methods were deleted in this PR. Since this line was already edited to prune `on_notify_defer

The `#[allow(non_snake_case)]` here previously decorated `mod EventLoop`, which this PR deletes; it now attaches through the intervening comments to `pub enum ContentsOrFd`, where it's dead (enums don't trigger `non_snake_case`, and `src/bundler/lib.rs` already has a crate-level `#![allow(non_snake_case, ...)]`). Per REVIEW.md — "delete dead code in the same PR that makes it dead" — line 44 should be removed along with the module.

This comment block (lines 104–111) still references `LinkerContext::any_loop_mut` and `on_complete`, both of which this PR removed from this code path — dispatch now goes through `crate::post::post`, and the identical block in `ParseTask.rs` was deleted. The fresh SAFETY comment at 112–113 already covers the invariant, so lines 104–111 can be dropped.