chore(msrv): feature-gate eframe/egui behind opt-in gui feature (#39)

## Summary

`eframe = "0.34"` (and the `egui` chain it pulls) raises MSRV above
1.85.0, which conflicts with the project's declared `rust-version =
"1.85.0"` and breaks the MSRV CI job.

Move `eframe` into a new opt-in `gui` feature so default + non-GUI
optional builds stay MSRV-clean. Users who want the native viewer build
with `--features gui` on a newer toolchain.

### Changes (4 files, +12/-3)

| File | Change |
|------|--------|
| `Cargo.toml` | `eframe = { ..., optional = true }`; add `gui =
["eframe"]` feature |
| `src/report/mod.rs` | cfg-gate `pub mod gui` + `pub use
gui::ReportGui` |
| `src/main.rs:472` | cfg-gate the `Commands::Gui` clap variant |
| `src/main.rs:1703` | cfg-gate the `Commands::Gui { .. }` match arm |
| `.github/workflows/rust-ci.yml` | MSRV check: `cargo check --features
signing,http` (was `--all-features`) |

### Why `--features signing,http` not `--all-features`

The MSRV job should test what users actually consume on the declared
MSRV. `signing` (ed25519-dalek 2.1) and `http` (ureq 3.3) are both
1.85-compatible. The new `gui` feature explicitly raises MSRV — building
it on 1.85 will fail, which is correct behaviour.

## Test plan

- [x] `cargo check` — default build OK
- [x] `cargo check --features signing,http` — the new MSRV check command
OK
- [x] `cargo check --features gui` — opt-in build still works
- [x] `cargo clippy --all-features --all-targets -- -D warnings` — clean
- [x] `cargo test --features signing,http --no-fail-fast` — all 17
suites pass
- [x] `cargo fmt --check` — clean
- [x] Signed commit, MPL-2.0 headers untouched

## Follow-up

Future improvement: add a `cargo check --features gui` job pinned to a
recent stable Rust so the GUI path stays buildable. Out of scope for
this PR (single-concern unbreakage).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

.github/workflows/rust-ci.yml

@@ -68,4 +68,6 @@ jobs:
         uses: Swatinem/rust-cache@779680da715d629ac1d338a641029a2f4372abb5 # v2
 
       - name: Check build
-        run: cargo check --all-features
+        # NB: the `gui` feature pulls eframe/egui which raise MSRV above 1.85.
+        # Verify the default + non-GUI optional features compile on MSRV.
+        run: cargo check --features signing,http

Cargo.toml

@@ -24,7 +24,7 @@ chrono = "0.4"
 filetime = "0.2"
 encoding_rs = "0.8"
 crossterm = "0.29"
-eframe = "0.34"
+eframe = { version = "0.34", optional = true }
 rayon = "1.12"
 blake3 = { version = "1.8", features = ["mmap"] }
 sha2 = "0.11"
@@ -39,6 +39,9 @@ ureq = { version = "3.3", optional = true }
 default = []
 signing = ["ed25519-dalek"]
 http = ["ureq"]
+# Native GUI viewer (eframe/egui). Opt-in because eframe raises MSRV above the
+# 1.85.0 baseline; default builds remain pure-CLI and MSRV-clean.
+gui = ["eframe"]
 
 [dev-dependencies]
 tempfile = "3.27"

src/main.rs

@@ -468,7 +468,8 @@ enum Commands {
         headless: bool,
     },
 
-    /// GUI review of a saved report
+    /// GUI review of a saved report (requires `--features gui` at build time)
+    #[cfg(feature = "gui")]
     Gui {
         /// Assault report JSON file
         #[arg(value_name = "REPORT")]
@@ -1700,6 +1701,7 @@ fn run_main() -> Result<()> {
             }
         }
 
+        #[cfg(feature = "gui")]
         Commands::Gui { report, headless } => {
             let content = read_report_bounded(&report)?;
             let assault_report: AssaultReport = serde_json::from_str(&content)?;

src/report/mod.rs

@@ -5,6 +5,7 @@
 pub mod diff;
 pub mod formatter;
 pub mod generator;
+#[cfg(feature = "gui")]
 pub mod gui;
 pub mod migration;
 pub mod output;
@@ -19,6 +20,7 @@ use std::path::Path;
 pub use diff::{format_diff, load_report};
 pub use formatter::{ReportFormatter, ReportView};
 pub use generator::ReportGenerator;
+#[cfg(feature = "gui")]
 pub use gui::ReportGui;
 pub use output::ReportOutputFormat;
 pub use tui::ReportTui;