feat: implement gloss-fetch network layer

Walks the `gloss-fetch--sources' registry in the order set by the
`gloss-fetch-sources' defcustom and aggregates per-source results into
the public `gloss-fetch-definitions' shape.

The Wiktionary REST fetcher GETs the page-definition endpoint, parses
JSON, walks only English (`en') entries, and HTML-strips each sense via
`libxml-parse-html-region'. A sense whose strip fails is dropped while
the source keeps its `:ok' status with N-1 entries.

The HTTP-status taxonomy is five values: `:ok', `:no-defs' (404 or no
English senses on a 200), `:rate-limited' (429), `:server-error' (5xx,
malformed JSON, schema mismatch, 4xx other than 404 or 429), and
`:unreachable' (nil from `url-retrieve-synchronously', or a signaled
error). The `:reason' string carries technical detail to *gloss-debug*
and never reaches the user.

libxml is probed once per session at first fetch. When absent, online
fetch is disabled package-wide and every call signals `user-error' with
the install hint. `url-retrieve-synchronously' is wrapped with the
`gloss-fetch-timeout' defcustom (default 5 seconds).

Tested with `make test'. 60 of 62 tests pass. The two pending failures
load Wiktionary fixtures via `gloss-test--load-wiktionary-fixture',
which is provided on a parallel branch and will pass once both branches
land. The implementation has been verified against the captured
fixtures end-to-end (anaphora returns 4 senses, SBIR returns 2,
matching the design's expected counts).

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