Fetch icons a stale cached set lacks, and scan Markdown

[dannote]

Fixes #2.

Cause. Cache.fetch_set/1 always prefers the copy of a set cached in priv/iconify/sets/. If that copy was downloaded before an icon was added to the collection, the compiler reports "Icon not found" and never refetches. That's why lucide:layer-arrow-up failed while Iconify.Fetcher.fetch_icon/2 worked: a fresh Lucide set (1,929 icons) has it.

Fix.

• PhoenixIconify.Cache.get_icons/3 takes icons from the cached set and fetches whatever it lacks one by one from the Iconify API. The compiler uses it.
• Unknown names are now reported as not found. Before, the batched Iconify.Fetcher.fetch_icons/2 raised on them through Set.get!.
• Markdown files are now scanned too. v0.3.5 added content/**/*.md to the default globs, but scan_file/1 skipped them, so icons in Astral Markdown needed extra_icons. Markdown goes through the HEEx scanner, which already falls back to line-by-line parsing when a file isn't valid HEEx.

Tests. A stale cached set test covers the issue's case with a fake fetcher (cached icon, icon only in the API, and an icon that exists nowhere). A Markdown test has an icon next to a code block containing <. mix ci passes (32 tests).

[dannote]

Updated:

• Markdown only with Astral. The default globs include Astral templates and Markdown (pages/**/*.{astral,md}, components/, layouts/, content/**/*.md) only when Astral is a dependency. Other projects scan just their Phoenix sources.
• Tests mirror lib/. The get_icons/3 tests now live in cache_test.exs.
• End-to-end tests are separate. test/e2e/phoenix_iconify/cache_test.exs reproduces New Lucide icons #2 against the real Iconify API. It's tagged :e2e, excluded from mix test, and runs with mix test.e2e.
• Needs iconify 0.3.1 (Leave unknown names out of fetch_icons instead of raising iconify#1). The e2e test showed that Iconify.Fetcher.fetch_icon/2 raised on unknown names. That's fixed in iconify, and this PR now requires ~> 0.3.1. Its dependencies resolve once that's published.

Against the fixed iconify (ICONIFY_PATH), mix test passes (34 tests) and so does mix test.e2e.