docs: describe the runtime public API surface

This adds crate-level and type-level Rustdoc to the runtime crate's core exported types so downstream crates and contributors can understand the session, prompt, permission, OAuth, usage, and tool I/O primitives without spelunking every implementation file.

Constraint: The docs pass needed to stay focused on public runtime types without changing behavior
Rejected: Add blanket docs to every public item in one sweep | larger churn than needed for a targeted docs pass
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: When exporting new runtime primitives from lib.rs, add a short Rustdoc summary in the defining module at the same time
Tested: cargo build --workspace; cargo test --workspace
Not-tested: rustdoc HTML rendering beyond  doc-test coverage

rust/crates/runtime/src/usage.rs

@@ -5,6 +5,7 @@ const DEFAULT_OUTPUT_COST_PER_MILLION: f64 = 75.0;
 const DEFAULT_CACHE_CREATION_COST_PER_MILLION: f64 = 18.75;
 const DEFAULT_CACHE_READ_COST_PER_MILLION: f64 = 1.5;
 
+/// Per-million-token pricing used for cost estimation.
 #[derive(Debug, Clone, Copy, PartialEq)]
 pub struct ModelPricing {
     pub input_cost_per_million: f64,
@@ -25,6 +26,7 @@ impl ModelPricing {
     }
 }
 
+/// Token counters accumulated for a conversation turn or session.
 #[derive(Debug, Clone, Copy, Default, PartialEq, Eq)]
 pub struct TokenUsage {
     pub input_tokens: u32,
@@ -33,6 +35,7 @@ pub struct TokenUsage {
     pub cache_read_input_tokens: u32,
 }
 
+/// Estimated dollar cost derived from a [`TokenUsage`] sample.
 #[derive(Debug, Clone, Copy, PartialEq)]
 pub struct UsageCostEstimate {
     pub input_cost_usd: f64,
@@ -51,6 +54,7 @@ impl UsageCostEstimate {
     }
 }
 
+/// Returns pricing metadata for a known model alias or family.
 #[must_use]
 pub fn pricing_for_model(model: &str) -> Option<ModelPricing> {
     let normalized = model.to_ascii_lowercase();
@@ -155,10 +159,12 @@ fn cost_for_tokens(tokens: u32, usd_per_million_tokens: f64) -> f64 {
 }
 
 #[must_use]
+/// Formats a dollar-denominated value for CLI display.
 pub fn format_usd(amount: f64) -> String {
     format!("${amount:.4}")
 }
 
+/// Aggregates token usage across a running session.
 #[derive(Debug, Clone, Default, PartialEq, Eq)]
 pub struct UsageTracker {
     latest_turn: TokenUsage,