…4407)

Closes denoland#30024

## Summary

Implements the object form of the npm `browser` field for `deno bundle
--platform browser`. The simple string form was already supported; the
mapped form was silently dropped.

```json
"browser": {
  "./bar.js": "./bar.browser.js",
  "crypto": false,
  "foo": "./shims/foo.js"
}
```

- relative-path keys remap the resolved file (`./bar.js` →
`./bar.browser.js`)
- bare-name keys remap an import specifier (`foo` → `./shims/foo.js`),
with `node:` prefix stripped before lookup so `import "node:crypto"`
matches a `"crypto"` key
- a value of `false` disables the module — the bundler substitutes an
empty stub

Esbuild's built-in browser-map handling is bypassed because Deno's
`on_resolve` plugin claims every resolve with filter `.*`, so we have to
apply the mapping ourselves.

## Where the logic lives

- **`libs/package_json`** — parses the object form into `browser_map:
Option<IndexMap<String, BrowserMapEntry>>` (`Replace(String) |
Disabled`). The string form keeps populating the existing `browser`
field.
- **`libs/node_resolver`** — substitution happens here, gated on the
existing `prefer_browser_field` flag (only set when bundling with
`--platform browser`). Hooks in `resolve`, `resolve_package`, and
`resolve_package_subpath_from_deno_module`. A new
`BrowserMapDisabledError` propagates `false` entries up to the caller.
- **`cli/tools/bundle/mod.rs`** — only catches `BrowserMapDisabledError`
from the resolver, returns a `\0deno-browser-disabled:<orig>` sentinel
path, and the `on_load` hook turns it into `module.exports = {}`.

## Test plan

- [x] New spec `tests/specs/bundle/browser_platform_map` covers all four
cases (relative remap, bare remap, bare disabled, transitive
within-package import) using new npm fixture
`@denotest/browser-field-map`
- [x] All 49 existing `bundle::*` spec tests still pass
- [x] `node_resolver` unit tests still pass
- [x] `--platform deno` bundles are unchanged (gate is
`prefer_browser_field`)