refactor: decompose large functions and remove dead code

- Decompose modify_request() into 7 single-responsibility helpers
- Decompose handle_http_over_tls(): extract read_full_request, dispatch_stream_events
- Promote connect_upstream/resolve_upstream to module-level functions
- Split standalone.rs (1238 lines) into 4 submodules:
  standalone/mod.rs, spawn.rs, discovery.rs, stub.rs
- Extract proto wire primitives into proto/wire.rs
- Remove 6 dead MitmStore methods
- Remove dead SessionResult, DEFAULT_SESSION, get_or_create
- Remove dead decode_varint_at, extract_conversation_id
- Clean all unused imports across 10 files
- Suppress structural dead_code warnings on deserialization fields

Warnings: 20 -> 0. All 43 tests pass.

src/mitm/proto.rs

@@ -37,6 +37,9 @@ use flate2::read::GzDecoder;
 use std::io::Read;
 use tracing::{debug, trace, warn};
 
+// Re-import the shared varint decoder under the name used throughout this module
+use crate::proto::wire::decode_varint as read_varint;
+
 /// A decoded protobuf field.
 #[derive(Debug, Clone)]
 pub enum ProtoValue {
@@ -260,26 +263,7 @@ fn looks_like_valid_message(fields: &[ProtoField], original_len: usize) -> bool
     }
 }
 
-/// Read a varint from a byte slice. Returns (value, bytes_consumed).
-pub fn read_varint(data: &[u8]) -> Option<(u64, usize)> {
-    let mut result: u64 = 0;
-    let mut shift = 0;
 
-    for (i, &byte) in data.iter().enumerate() {
-        if i >= 10 {
-            return None; // Too many bytes for a varint
-        }
-
-        result |= ((byte & 0x7F) as u64) << shift;
-        shift += 7;
-
-        if byte & 0x80 == 0 {
-            return Some((result, i + 1));
-        }
-    }
-
-    None
-}
 
 /// Search a decoded protobuf message tree for usage-like structures.
 ///