build: pure-Go (CGO_ENABLED=0) build via a klauspost-backed gozstd shim

dolthub/dolt/go pulls in two hard cgo dependencies — go-icu-regex (SQL
REGEXP) and gozstd (NBS zstd compression) — which forced a C toolchain +
ICU headers on every build. Both are now avoided so the default build is
pure Go and statically linkable:

- ICU: build with `-tags gms_pure_go`, selecting go-mysql-server's stdlib
  regexp fallback. Safe because this service never runs the SQL engine
  (it serves bare NBS stores and browses read-only), so it never
  evaluates SQL REGEXP.
- zstd: `replace github.com/dolthub/gozstd => ./third_party/gozstd-purego`,
  a pure-Go drop-in over klauspost/compress/zstd (already in the graph).
  It reproduces the nine gozstd symbols dolt references. dolt is
  unmodified.

dolt uses gozstd only in its NBS archive subsystem; this binary hits only
the decompress side at runtime (archive dictionary TRAINING is gc/
archive-writer code we never run — the shim implements it over klauspost
but panics on the trainer errors that only that off-path use could
trigger). zstd frames and dictionaries are standard-format, so
libzstd-authored archives decode correctly; the shim's tests prove this
by decoding plain and dictionary-compressed frames produced by the zstd
CLI (libzstd).

The Makefile now defaults to CGO_ENABLED=0 + -tags gms_pure_go (override
with `make CGO_ENABLED=1 GO_TAGS=` for the cgo variant). Verified:
CGO_ENABLED=0 build of ./..., all unit tests, the real-dolt-CLI
integration + spike suites, and the shim's libzstd-interop tests, all
green with no cgo.

rename module to sourcecraft.dev/bigbes/sr-ht-dolt; depend on sourcecraft sr-ht-core

build: source core-go from the instance fork

Replace git.sr.ht/~sircmpwn/core-go with git.srht.bigb.es/~bigbes/core-go
at c2c2f38 (upstream fdb3662 + newer upstream commits + the phoebe-lab S3
UnsignedPayload patch) - the code production actually runs. Requires
GOPRIVATE=git.srht.bigb.es to fetch. Full suite re-verified including the
remotesapi integration scenarios.

cmd: doltsrht and doltsrht-migrate binaries, module tidy

foundation: module, core domain, schema, spike-verified dolt interop

Phase 0 foundation for dolt.sr.ht (pure-Go Dolt hosting for SourceHut).

Dependency pins:
- core-go v0.0.0-20260520082310-fdb3662452dc: the production instance's
  submodule commit (fdb3662); must never be upgraded so token/config/crypto
  behavior matches the rest of the instance.
- dolthub/dolt/go v0.40.5-0.20260626152440-45335d44ad79: pseudo-version at the
  commit tagged v2.1.10 (45335d44), matching the installed dolt CLI. The /go
  submodule's latest tag (v0.40.4, 2021) does not interop with a modern CLI;
  matching the CLI commit guarantees a common NBS format (Format_DOLT) and
  remotesapi proto.
- gopkg.in/go-jose/go-jose.v2 v2.6.3: same JOSE version dolt's creds package
  signs EdDSA keypair JWTs with, avoiding a duplicate JOSE lib.
- grpc v1.79.3, logrus v1.8.3, lib/pq v1.10.9, chi/v5 v5.3.1, brant v0.5.1.

Contents:
- core/ pure domain: models, ValidateName/ParseRepoPath, the access matrix
  (Allowed + NotFoundForPrivate), full table-driven tests.
- schema.sql + migrations/0001_initial.sql (brant format).
- config.example.ini, contrib/dolt.sr.ht.conf (nginx), Makefile, scss/main.scss,
  static/logo.svg, README, .gitignore.
- internal/smoke: throwaway import-and-build check (deleted in Phase 3).

Spike (storage/spike_test.go, tag `spike`): bare NBS store via WriteEmptyRepo,
served by remotesrv on an ephemeral single port, round-tripped through the real
dolt CLI v2.1.10 (clone -> insert -> commit -> push -> re-clone -> verify rows).
Green, no version juggling needed beyond selecting the v2.1.10 commit. FS must
be rooted at the repos dir (LocalFilesysWithWorkingDir) so sealed chunk-URL
prefixes are clean relatives.

Build note: dolt pulls go-icu-regex (CGO), which needs ICU4C headers; on this
Mac set CGO_CPPFLAGS/CGO_LDFLAGS to icu4c@78 (documented in README).