feat: #set property accessors (write counterpart of #get)

A method `name :: (self: *T, value: V) #set { ... }` (or `=> expr;`) is the
write counterpart of a `#get` accessor: `obj.name = rhs` dispatches to it as
`obj.name(rhs)` when no real field matches. Plumbed parallel to `#get`:

- lexer/token `#set`; `FnDecl.is_set` + `Function.is_set`; parsed in the same
  marker slot as `#get` (no return type, exactly self + one value param).
- get+set coexistence: a setter registers/mangles/dispatches under an effective
  `name$set` name (`$` is illegal in sx identifiers, so unmistakable), keeping a
  same-name `#get` under the plain `name`. Resolution is declaration-order-
  independent: a plain read query picks the non-setter, a `name$set` write query
  picks the setter (accessorEffName / accessorNameMatches / structMethodFn).
- write dispatch in lowerAssignment via tryLowerPropertyAssignment: plain assign
  synthesizes `obj.name$set(rhs)`; compound `OP=` is get-modify-set and
  evaluates the receiver EXACTLY ONCE (bound to a synthetic local); read-only
  (#get-only) and write-only (#set-only + compound) emit clear diagnostics; a
  real field of the same name still wins. Multi-assign property targets dispatch
  the setter too (tryLowerPropertyStore, via a pre-lowered-Ref binding).

Payoff: List gains a `len` #set, so `xs.len = n` works; the `.items.len = N`
write workarounds in sched.sx + ui/* + platform/* revert to `xs.len = N`.

issues/0160 records an optional-chain interaction surfaced by the review (a
pre-existing `?T` value-optional read miscompile that blocks getter-through-`?.`).

examples/types/0198-types-set-property-accessor.sx

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+// `#set` property accessors — the write counterpart of `#get`. A method
+// `name :: (self: *T, value: V) #set { ... }` is invoked via field-assign
+// syntax (`obj.name = rhs`) rather than `obj.name(rhs)`. A property may carry
+// BOTH a `#get` and a `#set` of the same name (reads pick the getter, writes
+// pick the setter); compound assignment (`+=`) reads via `#get` then writes via
+// `#set`. Works on plain structs and generic-struct instances, as a multi-assign
+// target, and the compound form evaluates the receiver exactly once.
+#import "modules/std.sx";
+
+// get + set pair on the same name, with a scaling setter so we can see which
+// path fired.
+Temp :: struct {
+    raw: i64 = 0;
+    celsius :: (self: *Temp) -> i64 #get => self.raw;
+    celsius :: (self: *Temp, v: i64) #set { self.raw = v; }
+}
+
+// Generic instance: setter takes the type parameter as its value type.
+Box :: struct ($T: Type) {
+    slot: T;
+    val :: (self: *Box(T)) -> T #get => self.slot;
+    val :: (self: *Box(T), v: T) #set { self.slot = v; }
+}
+
+main :: () -> i64 {
+    t : Temp = .{};
+    t.celsius = 30;                         // setter
+    print("celsius={}\n", t.celsius);       // 30 (getter)
+    t.celsius += 5;                          // get-modify-set
+    print("after +=5: {}\n", t.celsius);    // 35
+    t.celsius *= 2;                          // get-modify-set
+    print("after *=2: {}\n", t.celsius);    // 70
+
+    b : Box(i64) = .{ slot = 1 };
+    b.val = 99;                              // setter (value type is T = i64)
+    print("box={}\n", b.val);                // 99
+    b.val -= 9;
+    print("box={}\n", b.val);                // 90
+
+    // Multi-assign with property targets — a swap proves all RHS values are
+    // evaluated before any setter fires.
+    p : Temp = .{ raw = 1 };
+    q : Temp = .{ raw = 2 };
+    p.celsius, q.celsius = q.celsius, p.celsius;
+    print("swap: p={} q={}\n", p.celsius, q.celsius);   // 2 1
+
+    // Compound assign through a property evaluates the receiver EXACTLY ONCE:
+    // a moving receiver reads and writes the SAME element (not two different ones).
+    g_idx = 0;
+    cells[0] = .{ raw = 10 };
+    cells[1] = .{ raw = 20 };
+    next_cell().celsius += 1;                 // reads & writes cells[0]
+    print("once: c0={} c1={} idx={}\n", cells[0].celsius, cells[1].celsius, g_idx);  // 11 20 1
+    return 0;
+}
+
+g_idx : i64 = 0;
+cells : [2]Temp = .[ .{}, .{} ];
+next_cell :: () -> *Temp {
+    cur := g_idx;
+    g_idx = g_idx + 1;
+    return @cells[cur];
+}

examples/types/expected/0198-types-set-property-accessor.exit

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+0

examples/types/expected/0198-types-set-property-accessor.stderr

@@ -0,0 +1 @@
+

examples/types/expected/0198-types-set-property-accessor.stdout

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+celsius=30
+after +=5: 35
+after *=2: 70
+box=99
+box=90
+swap: p=2 q=1
+once: c0=11 c1=20 idx=1