match against environment manifest version before using the default channel

alternative to #1095

Author: KristofferC

Cargo.lock

@@ -61,6 +61,8 @@ is-terminal = "0.4"
 path-absolutize = "3.1.0"
 numeric-sort = "0.1.5"
 regex = "1.10"
+toml = "0.8"
+expand-tilde = "0.6.1"
 
 [target.'cfg(windows)'.dependencies]
 windows = { version = "0.61.1", features = ["Win32_Foundation", "Win32_UI_Shell", "Win32_Security", "Win32_System_JobObjects", "Win32_System_Console", "Win32_System_Threading", "Services_Store", "Foundation", "Foundation_Collections", "Web_Http", "Web_Http_Headers", "Storage_Streams", "Management_Deployment"] }

Cargo.toml

@@ -156,10 +156,25 @@ The Julia launcher `julia` automatically determines which specific version of Ju
 1. A command line Julia version specifier, such as `julia +release`.
 2. The `JULIAUP_CHANNEL` environment variable.
 3. A directory override, set with the `juliaup override set` command.
-3. The default Juliaup channel.
+4. Automatic version selection based on the active project.
+5. The default Juliaup channel.
 
 The channel is used in the order listed above, using the first available option.
 
+### Project-based Version Selection
+
+When no explicit channel is specified via command line, environment variable, or directory override, Juliaup will automatically attempt to select an appropriate Julia version based on the active project's requirements.
+
+If a project is specified (via `--project`, `JULIA_PROJECT`, or `JULIA_LOAD_PATH`), Juliaup reads the project's `Manifest.toml` and uses the `julia_version` field to determine which Julia version to use:
+
+1. **Exact match**: If the exact version exists as a channel (e.g., `1.10.5`, `1.12.0-rc1`), it uses that version.
+2. **Prerelease versions**: If the version has a prerelease suffix (e.g., `1.12.1-DEV`, `1.13.0-rc1`) and no exact channel exists, it uses the corresponding nightly channel:
+   - Uses `X.Y-nightly` if available (e.g., `1.12-nightly` for `1.12.1-DEV`)
+   - Falls back to `nightly` otherwise
+3. **Future patch versions**: If the patch version is higher than any known release in that minor series (e.g., `1.10.99` when only `1.10.5` exists), it uses `X.Y-nightly`.
+4. **Future minor/major versions**: If the version is higher than any known release (e.g., `1.13.0` when only `1.12.x` exists), it uses `X.Y-nightly` if available, or `nightly` otherwise.
+5. **Default fallback**: If no project or manifest is found, it falls back to the default channel.
+
 ## Path used by Juliaup
 
 Juliaup will by default use the Julia depot at `~/.julia` to store Julia versions and configuration files. This can be changed by setting