docs(bf-4wz6v): document candidate cargo nextest timing flags

Add summary of timing flags identified in bf-52v1t research:

- Primary candidates: --message-format libtest-json variants for per-test timing
- Secondary candidates: --status-level and --final-status-level for slow detection
- Config: slow-timeout setting for threshold definition
- Excluded: --timings flag (build timing, not test timing)

Closes bf-4wz6v

notes/bf-mge0o-flags.md

@@ -0,0 +1,145 @@
+# Candidate cargo nextest Timing Flags
+
+**Bead:** bf-4wz6v  
+**Task:** Document candidate cargo nextest timing flags identified in bf-52v1t  
+**Date:** 2026-07-07  
+**Source:** notes/bf-52v1t.md
+
+## Summary
+
+This document summarizes the candidate timing-related flags from cargo nextest research (bf-52v1t), ready for local testing.
+
+## Primary Candidates (Per-Test Timing)
+
+### 1. `--message-format=libtest-json-plus`
+
+**Purpose:** Output test results in structured JSON format including detailed execution time per test  
+**Status:** Experimental  
+**Syntax:**
+```bash
+cargo nextest run --message-format=libtest-json-plus
+```
+
+**Expected output:** JSON stream with one object per test event, including `duration` field for execution time.
+
+**Use case:** Programmatic consumption of per-test timing data.
+
+---
+
+### 2. `--message-format=libtest-json`
+
+**Purpose:** Output in libtest-compatible JSON format (includes timing)  
+**Status:** Experimental  
+**Syntax:**
+```bash
+cargo nextest run --message-format=libtest-json
+```
+
+**Expected output:** Similar to libtest-json-plus but without nextest-specific metadata extensions.
+
+**Use case:** Compatibility with libtest JSON consumers.
+
+---
+
+### 3. `--message-format-version=<VERSION>`
+
+**Purpose:** Pin structured output format version for stable parsing  
+**Status:** Experimental  
+**Syntax:**
+```bash
+cargo nextest run --message-format=libtest-json-plus --message-format-version=1
+```
+
+**Expected behavior:** Ensures JSON schema remains consistent across nextest versions.
+
+**Use case:** Production parsing pipelines needing stability guarantees.
+
+---
+
+## Secondary Candidates (Slow Test Detection)
+
+### 4. `--status-level=slow`
+
+**Purpose:** Display tests that exceed the `slow-timeout` threshold during execution  
+**Status:** Stable  
+**Syntax:**
+```bash
+cargo nextest run --status-level=slow
+```
+
+**Expected behavior:** Prints test events when tests are marked "slow" (exceeding configured threshold).
+
+**Requires:** `slow-timeout` configured in `.config/nextest.toml` profile section.
+
+---
+
+### 5. `--final-status-level=slow`
+
+**Purpose:** Show slow test summary at end of run  
+**Status:** Stable  
+**Syntax:**
+```bash
+cargo nextest run --final-status-level=slow
+```
+
+**Expected behavior:** Prints summary of all tests that exceeded `slow-timeout` after completion.
+
+**Requires:** `slow-timeout` configured in `.config/nextest.toml` profile section.
+
+---
+
+## Configuration (Not a Flag, But Critical)
+
+### `slow-timeout` (config file only)
+
+**Purpose:** Define what constitutes a "slow" test  
+**File:** `.config/nextest.toml`  
+**Syntax:**
+```toml
+[profile.ci]
+slow-timeout = { period = "60s", terminate-after = 3 }
+```
+
+**Components:**
+- `period` - Threshold before marking test as "slow"
+- `terminate-after` - Hard timeout multiplier (period × N)
+
+**Default:** Tests exceeding `period` are tagged as "slow" for status-level flags.
+
+---
+
+## Excluded Flag (Not Per-Test Timing)
+
+### `--timings[=<FMTS>]`
+
+**Purpose:** Generate build timing reports  
+**Status:** Unstable  
+**Syntax:**
+```bash
+cargo nextest run --timings=json
+cargo nextest run --timings=html
+```
+
+**Why excluded:** Despite its name, this flag generates **build compilation timing** (how long cargo spent compiling), NOT per-test execution timing. This is a common point of confusion.
+
+**Scope:** Reports on crate/target compilation duration, not test runtime.
+
+---
+
+## Testing Priority Order
+
+Recommended testing sequence:
+
+1. **`--message-format=libtest-json-plus`** - Primary candidate for per-test timing data
+2. **`--message-format-version=1`** - Schema stability verification
+3. **`--status-level=slow`** - Slow test detection (requires config setup)
+4. **`--final-status-level=slow`** - End-of-run slow test summary
+5. **`--message-format=libtest-json`** - Compatibility verification
+
+---
+
+## Acceptance Criteria Status
+
+- ✅ **List of at least 3 candidate timing flags from bf-52v1t** (5 documented)
+- ✅ **Each flag has brief description of expected purpose** (provided)
+- ✅ **Document saved to notes/bf-mge0o-flags.md** (this file)