proto becomes bxcore, the one shared crate (M5b task 00)

Done the way the owner asked and the spike mapped: the crate and the
manifests renamed, then what the build reported fixed. Layers: 138
"proto unresolved" errors across 169 files; cargo fmt reordering
imports; six fixture paths built as strings in brokerd's and bxctl's
tests, which the compiler cannot see, and comments naming
crates/proto/; the crate-dependency check and its self-test. One more
the spike did not reach: a `\nproto =` key inside a printf format in
the self-test, which a word-boundary replacement skips because `n` and
`p` are both word characters.

make gate: gate: ok, 944 tests; no new entry in /tmp.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

crates/bxcore/src/log.rs

@@ -0,0 +1,74 @@
+//! Session log record types. One JSON object per line; assistant messages are stored verbatim.
+
+use serde::{Deserialize, Serialize};
+
+use crate::{CallId, DataClass, Epoch, Hash32, SessionId, Timestamp};
+
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(deny_unknown_fields)]
+pub struct ToolCall {
+    pub id: String,
+    pub name: String,
+    pub arguments: String,
+}
+
+/// What one completion cost. The same five numbers as `LogRecord::Usage`, without the time.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(deny_unknown_fields)]
+pub struct Usage {
+    pub cache_n: u64,
+    pub prompt_n: u64,
+    pub predicted_n: u64,
+    pub reasoning_tokens: u64,
+    pub thinking_capped: bool,
+}
+
+// JSON: {"type":"user","time":"…","content":"…"} ; the tag sits beside the fields; types are snake_case
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(tag = "type", rename_all = "snake_case", deny_unknown_fields)]
+pub enum LogRecord {
+    SessionStart {
+        time: Timestamp,
+        session: SessionId,
+        epoch: Epoch,
+        slot: u32,
+        baseline: Hash32,
+    },
+    User {
+        time: Timestamp,
+        content: String,
+    },
+    Assistant {
+        time: Timestamp,
+        content: Option<String>,
+        reasoning_content: Option<String>,
+        tool_calls: Vec<ToolCall>,
+    },
+    ToolResult {
+        time: Timestamp,
+        call: CallId,
+        tool_call_id: String,
+        content: String,
+        class: DataClass,
+        untrusted: bool,
+        truncated: bool,
+    },
+    Usage {
+        time: Timestamp,
+        cache_n: u64,
+        prompt_n: u64,
+        predicted_n: u64,
+        reasoning_tokens: u64,
+        thinking_capped: bool,
+    },
+    CacheLoss {
+        time: Timestamp,
+        expected: u64,
+        got: u64,
+    },
+    EpochEnd {
+        time: Timestamp,
+        next: Epoch,
+        summary: String,
+    },
+}