feat: Cargo sparse index (RFC 2789) + PyPI twine upload + PEP 691 (#113)

Cargo registry:
- Sparse index with config.json, prefix-based lookup (1/2/3/4+ char rules)
- cargo publish wire format (LE u32 lengths + JSON metadata + tarball)
- Version immutability with Cargo-compatible JSON error responses
- Dependency field mapping (version_req->req, explicit_name_in_toml->package)
- Case-insensitive crate name normalization across all endpoints
- Cache-Control headers on index (max-age=300) and downloads (immutable)

PyPI registry:
- twine upload via multipart/form-data with SHA-256 verification
- PEP 691 JSON API with Accept header content negotiation
- Hash fragment preservation in proxied links (PEP 503)
- Package name normalization per PEP 503

577 tests (up from 504), 0 failures, clippy clean.

Cargo.lock

@@ -176,6 +176,7 @@ dependencies = [
  "matchit",
  "memchr",
  "mime",
+ "multer",
  "percent-encoding",
  "pin-project-lite",
  "serde_core",
@@ -678,6 +679,15 @@ version = "1.0.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "34aa73646ffb006b8f5147f3dc182bd4bcb190227ce861fc4a4844bf8e3cb2c0"
 
+[[package]]
+name = "encoding_rs"
+version = "0.8.35"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "75030f3c4f45dafd7586dd6780965a8c7e8e285a5ecb86713e63a79c5b2766f3"
+dependencies = [
+ "cfg-if",
+]
+
 [[package]]
 name = "equivalent"
 version = "1.0.2"
@@ -1500,6 +1510,23 @@ dependencies = [
  "windows-sys 0.61.2",
 ]
 
+[[package]]
+name = "multer"
+version = "3.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "83e87776546dc87511aa5ee218730c92b666d7264ab6ed41f9d215af9cd5224b"
+dependencies = [
+ "bytes",
+ "encoding_rs",
+ "futures-util",
+ "http",
+ "httparse",
+ "memchr",
+ "mime",
+ "spin",
+ "version_check",
+]
+
 [[package]]
 name = "nonempty"
 version = "0.7.0"
@@ -1522,7 +1549,7 @@ dependencies = [
 
 [[package]]
 name = "nora-registry"
-version = "0.4.0"
+version = "0.5.0"
 dependencies = [
  "argon2",
  "async-trait",
@@ -2369,6 +2396,12 @@ dependencies = [
  "windows-sys 0.60.2",
 ]
 
+[[package]]
+name = "spin"
+version = "0.9.8"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6980e8d7511241f8acf4aebddbb1ff938df5eebe98691418c4468d0b72a96a67"
+
 [[package]]
 name = "spinning_top"
 version = "0.3.0"