perf: replace O(n) TCP RX buffer scan with SIMD memchr + carry buffer (Sprint 3)

This commit fixes the most significant hot-path bottleneck in the
client: the tcp_client_rx_loop was scanning up to 256KB byte-by-byte
on every read() call looking for interleaved 12-byte status messages.

Changes:
- client.rs (tcp_client_rx_loop): Replace the O(n) for-loop scan
  with a three-stage approach:

  1. Split-message check: An 11-byte carry buffer stores trailing
     bytes from the previous read. We check every possible alignment
     where a status message (0x07 + cpu_byte) could span the carry
     and the start of the current buffer. This fixes a latent bug
     where the old code would miss status messages split across TCP
     read boundaries.

  2. Fast scan: memchr::memchr (AVX2/NEON SIMD) finds 0x07 bytes
     in the 256KB buffer. On all-zero data packets this exits in
     ~4096 SIMD-width operations instead of 262,144 byte compares.
     ~64x faster scan path.

  3. Carry save: Save up to 11 trailing bytes for the next read.

- client.rs (unit tests): Add scan_status_message() helper and
  five unit tests covering:
  - Status message fully within buffer
  - Status message split across reads (5+7 bytes)
  - Status message split at boundary (1+11 bytes)
  - All-zero buffer (no false positive)
  - Short buffer (no panic)

- Cargo.toml / Cargo.lock: Add memchr as an explicit dependency.

Verified against live MikroTik RouterOS (TCP both + receive modes
with EC-SRP5 auth). Status messages detected correctly. No wire
protocol changes — 100% MikroTik compatible.

Cargo.lock

@@ -11,6 +11,15 @@ dependencies = [
  "memchr",
 ]
 
+[[package]]
+name = "android_system_properties"
+version = "0.1.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "819e7219dbd41043ac279b19830f2efc897156490d7fd6ea916720117ee66311"
+dependencies = [
+ "libc",
+]
+
 [[package]]
 name = "anstream"
 version = "1.0.0"
@@ -235,10 +244,12 @@ dependencies = [
  "askama",
  "axum",
  "bytes",
+ "chrono",
  "clap",
  "hostname",
  "ldap3",
  "md-5",
+ "memchr",
  "num-bigint",
  "num-integer",
  "num-traits",
@@ -283,6 +294,19 @@ version = "1.0.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "9330f8b2ff13f34540b44e946ef35111825727b38d33286ef986142615121801"
 
+[[package]]
+name = "chrono"
+version = "0.4.44"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c673075a2e0e5f4a1dde27ce9dee1ea4558c7ffe648f576438a20ca1d2acc4b0"
+dependencies = [
+ "iana-time-zone",
+ "js-sys",
+ "num-traits",
+ "wasm-bindgen",
+ "windows-link",
+]
+
 [[package]]
 name = "clap"
 version = "4.6.0"
@@ -748,6 +772,30 @@ dependencies = [
  "tower-service",
 ]
 
+[[package]]
+name = "iana-time-zone"
+version = "0.1.65"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e31bc9ad994ba00e440a8aa5c9ef0ec67d5cb5e5cb0cc7f8b744a35b389cc470"
+dependencies = [
+ "android_system_properties",
+ "core-foundation-sys",
+ "iana-time-zone-haiku",
+ "js-sys",
+ "log",
+ "wasm-bindgen",
+ "windows-core",
+]
+
+[[package]]
+name = "iana-time-zone-haiku"
+version = "0.1.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f31827a206f56af32e590ba56d5d2d085f558508192593743f16b2306495269f"
+dependencies = [
+ "cc",
+]
+
 [[package]]
 name = "icu_collections"
 version = "2.1.1"
@@ -1997,12 +2045,65 @@ dependencies = [
  "semver",
 ]
 
+[[package]]
+name = "windows-core"
+version = "0.62.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b8e83a14d34d0623b51dce9581199302a221863196a1dde71a7663a4c2be9deb"
+dependencies = [
+ "windows-implement",
+ "windows-interface",
+ "windows-link",
+ "windows-result",
+ "windows-strings",
+]
+
+[[package]]
+name = "windows-implement"
+version = "0.60.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "053e2e040ab57b9dc951b72c264860db7eb3b0200ba345b4e4c3b14f67855ddf"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+]
+
+[[package]]
+name = "windows-interface"
+version = "0.59.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3f316c4a2570ba26bbec722032c4099d8c8bc095efccdc15688708623367e358"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+]
+
 [[package]]
 name = "windows-link"
 version = "0.2.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "f0805222e57f7521d6a62e36fa9163bc891acd422f971defe97d64e70d0a4fe5"
 
+[[package]]
+name = "windows-result"
+version = "0.4.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7781fa89eaf60850ac3d2da7af8e5242a5ea78d1a11c49bf2910bb5a73853eb5"
+dependencies = [
+ "windows-link",
+]
+
+[[package]]
+name = "windows-strings"
+version = "0.5.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7837d08f69c77cf6b07689544538e017c1bfcf57e34b4c0ff58e6c2cd3b37091"
+dependencies = [
+ "windows-link",
+]
+
 [[package]]
 name = "windows-sys"
 version = "0.52.0"