chore(build): pin rustc 1.98.1, rust-lld, and the README's toolchain claim

The compiler moves into rust-toolchain.toml so the repository, rustup and both
workflows read one file; the MSVC linker is replaced by the rust-lld that ships
inside every toolchain, which halves the fat-LTO relink (112 s -> 56 s per
edit->build cycle). The README named 1.80+ for a tree that needs 1.98.

.cargo/config.toml

@@ -12,3 +12,20 @@ linker = "rust-lld"
 # artifact still starts (`--version` and `--legend` both exit 0 on the runner).
 [target.x86_64-win7-windows-msvc]
 linker = "rust-lld"
+
+# Rejected here, with the reason, so nobody re-tries them:
+#
+#   embed-metadata = false  Cargo 1.98.1 reports the key as unused — `cargo
+#                           metadata` prints "unused config key" on every
+#                           invocation, and it is part of the CI cache key, so a
+#                           stray key would bust the runners' caches for nothing.
+#                           The real lever is the nightly-only
+#                           `-Z no-embed-metadata` (cargo 1.98.1 lists it, but
+#                           stable rejects every `-Z` and neither installed
+#                           nightly knows the rustc option). Measured for scale:
+#                           a clean dev build is 1078 MiB, of which 434 MiB is
+#                           `.rlib` and 165 MiB `.rmeta` — the win would be a
+#                           fraction of that, and it cannot ship on the pinned
+#                           toolchain.
+#   sccache, mold           no benefit at one codegen unit with fat LTO, and
+#                           mold is ELF-only while these rows link with rust-lld.

README.md

@@ -298,7 +298,9 @@ NFQWS_EXTRA_ARGS="$MODE_ALL"
 
 ## 🏗️ Сборка из исходников
 
-Для самостоятельной сборки потребуется установленный Rust (версии 1.80+):
+Для самостоятельной сборки нужен Rust: версия закреплена в `rust-toolchain.toml`
+(сейчас 1.98.1), и `rustup` поставит её сам при первой сборке — ничего выбирать не надо.
+Если `rustup` не используется, компилятор должен быть не старее версии из этого файла.
 
 ```bash
 git clone https://github.com/Runnin4ik/dpi-detector.git -b rust

rust-toolchain.toml

@@ -4,9 +4,10 @@
 # "stable" meant on the day a runner picked it up.
 #
 # `profile` and `components` are exactly what CI needs: clippy is a required
-# check (AGENTS.md §4). rustfmt is deliberately not here — the sources are
-# formatted by hand (§3.7) — and rust-analyzer, rust-docs and the extra targets
-# are a contributor's own business (`rustup component add`).
+# check (AGENTS.md, "Testing & QA"). rustfmt is deliberately not here — the
+# sources are formatted by hand (AGENTS.md, "Development Commands") — and
+# rust-analyzer, rust-docs and the extra targets are a contributor's own
+# business (`rustup component add`).
 #
 # Three rows of the release matrix cannot use this pin and name their toolchain
 # on the command line instead: win7 and the two MIPS targets build std from