Cache parsed tiles on disk, and make the map's cost an operator setting (#133)

[CaYatur]

Cache parsed tiles on disk, and make the map's cost an operator setting (#133)

Measured on a real Paper 1.21.6 world before writing anything. One 5.4 MB region
of 811 chunks and 19 769 sections:

total 180ms | decompress 39ms | nbt parse 141ms

...before any surface extraction, and repeated every time the app starts,
because #119's cache was a Map in memory holding twelve regions. The disk half
was in that issue's scope and I only built the memory half. This is the rest.

The encoded form of a region is about 1 MB, gzips in 2-3 ms and gunzips in about
one. So a region now costs a few hundred milliseconds once, and single-digit
milliseconds forever after — parsed again only when the server rewrites it.

shared/tileCache.ts holds the format, pure, because this is where a mistake is
silent: a decoder that mis-reads its own file produces a plausible map rather
than an error, and then serves it until somebody deletes the cache by hand.
Fixed-width columns rather than a hand-rolled packing — the caller gzips it, and
gzip finds the runs better than a scheme I could get wrong.

The key carries a format version as well as the mtime. The mtime says the
world has not changed, which is true and beside the point when what changed is
how we draw it: without the version, every cache written before the colour and
foliage work would go on serving the old picture forever, and an operator
staring at it has no way to connect it to an update. Changing the colours, the
foliage rule or the shading and not bumping that number is the bug this field
exists to prevent.

Per-server settings, because a box running twenty servers and a laptop
running one want different answers:

• Cache tiles on disk — on by default; this is the one that matters.
• Regions in memory — the working set.
• Delay between parses — the brake that keeps the main thread responsive.
• Cache limit — oldest evicted first, plus a way to clear it outright.

They live on the server's own config so they survive a restart and apply to
every surface, and they are clamped on the way IN rather than on the way out: a
value only fixed when read is still a wrong number in the file, and every one of
these is a way to hang the process — a parse gap of zero removes the brake, a
memory limit of a million holds a whole world resident.

Asserted: the codec round-trips colours, heights across the full -64..319 range,
the transparent columns that must stay transparent, and structure marks with
their ids. A cache from an older format version is refused rather than decoded.
Wrong magic, an empty buffer, three different truncations and a chunk count that
runs past the end all answer null rather than throwing or returning half a map.
Proved failable — encoding transparent columns as present gives
FAIL - colour 0 of chunk 0: 0 vs -1, which is the void painted over every
ungenerated gap.

[Copilot]

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[CaYatur]

Self-review: "clear cache" wiped every server, and the panel had none of this

The clear button cleared everyone's. Cache files were named by a hash of the
region path alone, so there was nothing in a filename to say which server it
belonged to — and a hash cannot be reversed. "Clear this server's cache" deleted
every cached region on the machine while claiming to clear one. The server id is
a visible prefix now, so the question is answerable from the filenames.

The sweep ran after every region. It stats every file in the cache
directory, so writing a region meant a directory scan — a cost inside the thing
that exists to remove cost. It runs once per 32 MB added instead.

And I put the dials only in the desktop, one PR after merging the three maps
and one after re-splitting them. The panel is the admin surface an operator
reaches from anywhere, and this is a persisted server setting rather than a view
preference, so it belongs there too: GET/POST /servers/{id}/map/perf and
DELETE /servers/{id}/map/cache, all on settings, with the same controls in
the map tab.

The panel shows what was stored, not what was typed. The server clamps, and
a field that keeps displaying a refused number is lying about the setting.