csharp: emit member-access read/write edges

[pbednarcik]

C# property, field, and constant accesses emitted no edges of any kind — find_usages on any C# property answered with its declaration and nothing else. On a production C# codebase, 6 of 28,773 field nodes had any non-structural in-edge; every property looked unused no matter how often the code read or wrote it.

This brings C# to parity with the PHP and Go extractors through the existing pipeline: each access emits EdgeReads (EdgeWrites in assignment position — simple and compound assignment, ++/--, with element accesses climbed so p.Items[0] = v writes Items) to unresolved::*.<member>, the shape resolveFieldRef already consumes. Receiver typing rides the same ladder as calls: this/base from the enclosing declaration, locals from the tenv, known types for static accesses, chains through the chain typer. The resolver needed zero changes.

One rule PHP does not need: dotted namespace qualification parses as nested member accesses in this grammar, so System.Threading.Tasks.Task.CompletedTask would shower the graph with reads of Threading/Tasks. An access with no receiver evidence emits only at the outermost link of its chain — with one refinement: a receiver that names a declared parameter or local is a value, not a namespace, so its chained reads survive even untyped.

Documented v1 boundaries: parameter and field receivers emit untyped edges (binding falls to the resolver cascade, same contract as calls); conditional accesses are read-only; object-initializer assignments and attribute-argument reads don't emit.

Tests: extractor spec covering read/write/compound/this/static-const/conditional/chained shapes plus the call-position exclusion and the namespace-noise rule; resolver spec binding a typed read and write to the receiver type's property at ≥0.9 over a same-named decoy. Written first, watched fail. All suites at their pre-existing Windows baselines.

Validated live on a production C# codebase: 108,696 read + 9,978 write edges where there were none. On the counted fixture repo, an authored ledger (exactly 3 reads + 1 write of one property) pins as counted battery cells, green twice.

Stacked on #481 (which takes csharp extractor version 7); this PR bumps 7→8.