Restrict crate-internal items to pub(crate) and remove dead code it exposes (#31254)

## What

Three commits, one sweep:

1. **`pub` → `pub(crate)` for 1,298 items across 105 files.** Applied
mechanically via `cargo fix` driven by the `unreachable_pub` lint, which
only fires on items that are provably unreachable from outside their
defining crate — so the demotion cannot change what compiles. Two items
(`addrinfo`/`freeaddrinfo` re-exports in `bun_dns`) were reverted by
hand because a chained `pub use` re-exports them.

2. **Remove the dead AST JSON-serialization facility.** The `js_ast`
`jsonStringify` port — `JsonWriter`/`BindingJsonWriter` traits,
`Serializable` carrier structs, 26 `json_stringify` methods,
`Ast::to_json` — has zero trait implementors and zero callers. The only
`.json_stringify()` call sites in the tree are `JSValue`'s native
`JSON.stringify`, which is unrelated.

3. **Remove six crates nothing depends on**, plus the data files
vendored under them:
- `bun_unicode` — empty `lib.rs`, no consumers, but carried the entire
vendored Unicode Character Database (~70k lines of `.txt`)
- `bun_libarchive_sys` / `bun_picohttp_sys` — superseded duplicates;
`bun_libarchive` / `bun_picohttp` carry their own bindings and are what
everything actually uses
   - `bun_meta`, `bun_url_jsc` — zero callers
- the `bun_node-fallbacks` crate wrapper (the `.js` fallback modules
stay; codegen bundles those directly)
   - `crossbeam` from `[workspace.dependencies]` (no member declares it)

After this, every workspace crate except the two standalone build roots
(`bun_codegen`, `bun_shim_impl`) has at least one in-workspace
dependent.

## Why

`dead_code = "deny"` is already enforced workspace-wide, but it treats
every `pub` item as an external API root and never analyzes it. In a
workspace where every crate is an internal implementation detail of one
binary, that exemption swallows real dead code. Demoting crate-internal
items to `pub(crate)` puts them back under the lint — that is what
surfaced the dead serialization facility.

## Verification

- `cargo check --workspace --lib --bins` clean under the workspace's
normal lint set
- Full debug build links and runs
- `bundler_edgecase` 103 pass / 0 fail, `bundler_browser` 12 pass / 0
fail (node-fallbacks path), `bun-add` failures are pre-existing on main
(registry config leak in the test environment, identical signature with
an unmodified build)

```
179 files changed, 1,522 insertions(+), 84,970 deletions(-)
```