Preserve symlinks when staging, and link node_modules into web builds

[arcaputo3]

Stack 2/N. Base is agent/bun-140-toolchain (#14), not main.

The bug

BunScalaJSWebModule could not resolve any npm dependency.

Every tree copy in the plugin walks with os.walk (followLinks = false) but branches on os.isDir / os.copy (followLinks = true). A node_modules symlink — which ensureLinkedWorkspace plants in the link report — is therefore misclassified as a directory:

• BunWebSupport.copyContents called os.makeDir.all, producing an empty node_modules in the staged web build. bun build index.html then had nothing to resolve @JSImported packages against.
• BunToolchainModule.copyWorkspace called os.copy.over, which follows the link and deep-copies the entire resolved tree — O(node_modules) on every compileExecutable, bundle, and test workspace — and throws outright on any broken link, which is routine for the .bin shims of skipped optional dependencies.

Both verified with standalone os-lib repros before fixing.

The fix

BunToolchainModule.copyTree recreates symlinks instead of resolving them, and dispatches on os.isLink before os.isDir(followLinks = false). copyWorkspace, copyPathRefs, and both BunWebSupport copiers route through it.

BunScalaJSWebModule now stages node_modules explicitly — mirroring BunTypeScriptWebModule.prepareWebStage, which already did this and was correct — rather than hoping the link survives a copy.

Staging tasks now declare the install they link into. Mill's filesystem checker rejects reading a dest the task doesn't depend on, so recreating the symlink surfaced a dependency that was always real but never declared. The old deep-copy slipped past the checker only because os.copy checks the source path (inside the link report, a legal read) rather than the resolved target. Affected: BunScalaJSModule.compileExecutable/compileExecutables, BunTypeScriptModule.bundle/compileExecutable/compileExecutables, BunWorkersModule.bundleWorkers.

Also in scope, same files:

• htmlEntries is split into a pure resolver and materializeHtmlEntries. dev() and bundle called the writing version against a staging task's already-cached dest.
• BunTypeScriptWebModule.webDevelopmentStage and webProductionStage were byte-for-byte identical; collapsed into one webStage, so a production build no longer runs the same staging work twice.

Proof the test catches it

scalajs-web had no npm dependencies — which is precisely why CI was blind. It now imports lodash through @JSImport, so the linked output carries a real npm import.

Verified by stashing the source fix and running the fixture against the parent commit:

X BunScalaJSIntegrationTests.web bundle includes HTML CSS and JavaScript
X BunScalaJSIntegrationTests.web stage resolves npm dependencies
  os.SubprocessException: Result of .../bun...: 1

With the fix, both pass, stage/node_modules is a link, and lodash is inlined into the bundle rather than left as a bare import.

Verification

• Unit: 67 passing (was 57). New CopyTreeTests covers symlinked directories, broken links, exclusion, symlinked files, and empty directories; BunWebSupportTests covers htmlEntries purity and single-materialization.
• Integration: 37 passing.

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