Brings in #18513 (BatchDownloadProgress and the download_with_resume
progress callback) and extends the plumbing one level:
download_from_mirrors passes a progress callback to both target kinds,
Source/ConvertedLibrary.download thread it through, and the library
wave prefetch draws one combined bar sized by parallel HEAD requests
(no sizes, no bar). The thread-local bar suppression this replaces is
removed; a batch caller passing a tracker also owns the messaging, so
the per-file Downloading line only prints for solo downloads.
The walk now drains its frontier, prefetches the wave's registry
archives with a small thread pool (deduped by URL, progress bars
suppressed per thread since parallel bars would interleave), and then
processes the wave sequentially; the sequential download() call stays
authoritative so failures surface with resume exactly as before. Same
approach as the espidf tool prefetch in #18513.
The typed IncompatiblePlatform exception, dependency_is_usable,
warn_properties_depends, the lex_build_flags helper (espidf switches to
it), the normalize_dependencies validation, the manifest shape check,
the component-level drop-warning split, and the SCons case-sensitive
.C/.C++ suffixes all harden the shared converter independently of the
arduino backend, so they belong in this PR; the provides hook and the
version-less reconciliation stay with the backend that needs them.
J. Nick Koston