ci+docs: revert cross-build cache key to v2 (warm); document dev default in user docs

- cross-build-test.yml: revert xlings cache key v3->v2. The v3 bump forced a COLD
  ~/.xlings, whose first-time toolchain install 127'd; v2 reuses the warm cache
  the passing e2e/ci-linux jobs already use (has 0.0.75 + the toolchain). The
  original 'version 0.0.75 not found' poison is moot now 0.0.75 is fully
  propagated. The self-host bootstrap is otherwise unrelated to this PR's code
  (e2e passed; step 6 bootstrap succeeded).
- docs/05-mcpp-toml.md (+ zh): document the new convention — bare `mcpp build`
  uses dev (-O0 -g); release is opt-in via --release/--profile release; --dev
  shorthand; [build].default-profile per-project default (and the distribution
  note).

Author: Sunrisepeak

.github/workflows/cross-build-test.yml

@@ -74,9 +74,9 @@ jobs:
         uses: actions/cache@v4
         with:
           path: ~/.xlings
-          key: xlings-${{ runner.os }}-v3-${{ hashFiles('.xlings.json') }}
+          key: xlings-${{ runner.os }}-v2-${{ hashFiles('.xlings.json') }}
           restore-keys: |
-            xlings-${{ runner.os }}-v3-
+            xlings-${{ runner.os }}-v2-
 
       - name: Install qemu-user-static
         run: |

docs/05-mcpp-toml.md

@@ -411,8 +411,16 @@ cxxflags = ["-fno-plt"]
 ldflags  = []
 ```
 
-- Selection: `mcpp build --profile <name>` (and `mcpp test --profile <name>`, which builds the
-  code-under-test plus the test binaries under that profile), defaulting to `release`.
+- Selection & default: a bare `mcpp build` uses the **`dev`** profile (`-O0 -g`) — the
+  mainstream convention (cf. Cargo/Meson/CMake/Zig/Bazel). **Release is opt-in:**
+  `mcpp build --release` (shorthand) or `--profile release`. `--dev` is the explicit
+  shorthand for dev. Same applies to `mcpp test --profile <name>` (builds the
+  code-under-test plus the test binaries under that profile).
+- **Per-project default** — `[build].default-profile = "<name>"` (alias: `profile`) sets
+  the project's own default when no flag is passed. The typical use is a tool/library
+  that should build optimized by default: `[build] default-profile = "release"`. Precedence:
+  `--profile`/`--release`/`--dev` flag **>** `[build].default-profile` **>** global `dev`.
+  (A project that defaults to dev should pass `--release` when producing a distributable.)
 - Built-in profiles: `release` (-O2) / `dev`, `debug` (-O0 -g) / `dist` (-O3 + strip;
   **LTO is not enabled by default**). `[profile.<built-in name>]` can override a
   built-in definition wholesale.

docs/zh/05-mcpp-toml.md

@@ -389,8 +389,14 @@ cxxflags = ["-fno-plt"]
 ldflags  = []
 ```
 
-- 选择:`mcpp build --profile <name>`(以及 `mcpp test --profile <name>`,会让被测代码与测试二进制
-  都在该 profile 下编译),默认 `release`。
+- 选择与默认:裸 `mcpp build` 走 **`dev`** 档(`-O0 -g`)——主流惯例(参照
+  Cargo/Meson/CMake/Zig/Bazel)。**release 为 opt-in:** `mcpp build --release`(短写)或
+  `--profile release`;`--dev` 是 dev 的显式短写。`mcpp test --profile <name>` 同理
+  (被测代码与测试二进制都在该 profile 下编译)。
+- **项目级默认** —— `[build].default-profile = "<name>"`(别名 `profile`)设置该项目在不带
+  flag 时的默认。典型用途是"以发布优化为常态"的工具/库:`[build] default-profile = "release"`。
+  优先级:`--profile`/`--release`/`--dev` flag **>** `[build].default-profile` **>** 全局 `dev`。
+  (默认 dev 的项目在产出可分发物时应显式 `--release`。)
 - 内置档案:`release`(-O2)/ `dev`、`debug`(-O0 -g)/ `dist`(-O3 + strip;
   **不默认开 lto**)。`[profile.<内置名>]` 可整体覆盖内置定义。