feat(aura-std): Session — Frankfurt bars_since_open (DST-aware)

Maps ctx.now() (epoch-ns UTC) to a Frankfurt session bar index: emits
bars_since_open:i64 = (local wall-clock minutes past the 09:00 open) /
period_minutes, in tz-aware DST-correct local time. So local 09:45 reads 3 in
both CEST (UTC+2) and CET (UTC+1) — the close-instant convention (spec 0050
§4.1). Trigger is an f64 Any input (value ignored, wired from close15) so the
node fires once per completed bar. Open/tz/period are baked structural config,
not scalar params (a timezone is not a scalar, C11). Admits chrono/chrono-tz to
aura-std — vetted DST math, never hand-rolled (per-case dep policy). Last node;
build-step 7 of milestone 'Strategy node vocabulary I'.

closes #90

Cargo.lock

@@ -102,6 +102,8 @@ name = "aura-std"
 version = "0.1.0"
 dependencies = [
  "aura-core",
+ "chrono",
+ "chrono-tz",
 ]
 
 [[package]]
@@ -181,6 +183,16 @@ dependencies = [
  "windows-link",
 ]
 
+[[package]]
+name = "chrono-tz"
+version = "0.10.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a6139a8597ed92cf816dfb33f5dd6cf0bb93a6adc938f11039f371bc5bcd26c3"
+dependencies = [
+ "chrono",
+ "phf",
+]
+
 [[package]]
 name = "cipher"
 version = "0.4.4"
@@ -546,6 +558,24 @@ dependencies = [
  "hmac",
 ]
 
+[[package]]
+name = "phf"
+version = "0.12.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "913273894cec178f401a31ec4b656318d95473527be05c0752cc41cdc32be8b7"
+dependencies = [
+ "phf_shared",
+]
+
+[[package]]
+name = "phf_shared"
+version = "0.12.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "06005508882fb681fd97892ecff4b7fd0fee13ef1aa569f8695dae7ab9099981"
+dependencies = [
+ "siphasher",
+]
+
 [[package]]
 name = "pin-project-lite"
 version = "0.2.17"
@@ -689,6 +719,12 @@ version = "0.3.9"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "703d5c7ef118737c72f1af64ad2f6f8c5e1921f818cdcb97b8fe6fc69bf66214"
 
+[[package]]
+name = "siphasher"
+version = "1.0.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8ee5873ec9cce0195efcb7a4e9507a04cd49aec9c83d0389df45b1ef7ba2e649"
+
 [[package]]
 name = "slab"
 version = "0.4.12"